[dash] Switch to fdio_spawn_etc

Most codepaths that create processes should use fdio_spawn rather than
launchpad directly.

Test: Manually verified that the shell continues to work.
Change-Id: I0e9b21f17ac7b21b92a5bedd5bacf121dee8ff02

Revert "[dash] Switch to fdio_spawn_etc"

This reverts commit 157ba5410b36a86bda605ee77e6486c542d8c765.

Reason for revert: Breaks shell in zedboot

Test: "fx full-build && fx mkzedboot" with and without this change (and https://fuchsia-review.googlesource.com/c/zircon/+/169514)
Before this change, dash fails to launch processes.
With this change, dash can launch processes.

Change-Id: I6943cb164e127feafd798df632bdae68657abac0

[dash] Switch to fdio_spawn_etc

Most codepaths that create processes should use fdio_spawn rather than
launchpad directly.

Test: Manually verified that the shell continues to work.
Change-Id: I820b2079fba3123b955ac1071a81a09aa778dcfd

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[dash] enable ctrl-c in interpreters running under a pty

Change-Id: Ifc5fde187ac9aefd735b94099f5551512381020d

[dash] enable ctrl-c in interpreter (Revert)

This reverts commit df2e3a471711d219e6b0d217fe443cfe5ddde6e5.

Reason for revert: Causes breakages with commands run via netcmd.

Original change's description:
> [dash] enable ctrl-c in interpreter
>
> Change-Id: I4f4b7ab18ab79e3b30db008d6b98f6850ad53b67

TBR=jeffbrown@google.com,abarth@google.com,steveaustin@google.com

Change-Id: I82f25fe8f25324bce4ff8cb30952bb1826066848
No-Presubmit: true
No-Tree-Checks: true
No-Try: true

[dash] enable ctrl-c in interpreter

Change-Id: I4f4b7ab18ab79e3b30db008d6b98f6850ad53b67

[fdio][launchpad] Open CWD relative to root, using PWD env variable

With fallback path to PWD = "/", if no environment variable
named PWD is provided.

This improves the launchpad API by reducing the risk that a client
could unintentionally provide a CWD handle to a subprocess
which exists outside the subprocess' sandboxed root.

Change-Id: Icc9cfa002d7036e2de2b7775ae10cc04dba13f4d

[dash] Fix error reporting from process_launch

Before this, an int in the style of unix process exit() values (0
success, low numbers or high 120s different kind of bad, etc.) was
returned, but logged as though it were a zx_status_t. This tweaks
process_launch to return both styles and to print the more informative
zx_status_t to stderr, with $? still set correctly to the unix style.

Change-Id: I66cb615b2b83318829029f506f1e0f8156051eb4

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[procargs][mxio][launchpad] Remove MXIO_ROOT

There are no longer any clients of this startup handle. This patch keeps some
of the client machinery in mxio for devmgr. It will be removed in a future
patch.

Change-Id: I4eccd4a19a616d4847d9ab1ac07b9d00b62ad5ef

[dash] Migrate to the new MX_ error codes

Change-Id: I82e4ba00bc1cbe3609f74781b505d6476a7b2365

[dash][run-vc] Remote ApplicationEnvironment code

Now that we use namespaces in Fuchsia-land, we don't need the
ApplicationEnvironment-related code anymore.

Change-Id: Iae4181bd72efa63b59d97c46c61994644aac7193

[dash] Teach dash about launchpad error messages and mx_status_t names

Launchpad makes such nice error messages. Let's use them.

Change-Id: Icf6c68ec4395dd19bd8da2b64aef2b4d36123556

[processargs] tidy up naming and numbering

This cleans stuff up ahead of the addition of string tables for
the upcoming PA_NS_DIR and PA_NS_REQ types and the eventual
flattening of PA_MXIO_* to a single type, using subtypes for
protocol.

- Align names with current-day usage
- Organize numbering / ordering around subsystem usage
- Reduce "type" field to 8 bits
- Reserve space for "subtype" field
- Use PA_ namespace and shorter a lot of very long names

Change-Id: I5e59f412c3b9bd79d90f921a248a6d98d4b703bc

[dash] Pass variable definitions to subshells

Fixes the behavior of passing variables to subshells in the
environment, which improperly exported them. Now, we pass the
variables in a symbol table that is part of the context
provided to a subshell in a handle.

Change-Id: Ie4809f8f821100544920cf0b5040af9ea3f21aca

[dash] Pass command-line args to subshells.

Change-Id: I1db939d69bd71154e96cb533386f1380279daa09

[dash] Subshells inherit function definitions

Pass function definitions to subshells. To accomplish this,
instead of passing a single command to the subshell we pass a
list of commands, with the function definitions as the first
items in the list.

Change-Id: I0f9dd51e6aaa6bbcfac4acbb83003900fd6d8a72

[magenta] Migrate Magenta's uses of mxio_get_startup_handle

Leave the compatibility shim in place until Fuchsia-at-large migrates.

For MG-228: "get startup handles should not live in mxio"

Change-Id: I5bf53388585520bf7d4b5acc31902bfc7797489d

[dash] tidy up usage of new launchpad api

Change-Id: I1bb96a78e1514d1cf4783a6f4baadab6e5e97ed1

[syscalls] update callers to mx_handle_wait_*() and deprecate

Change-Id: I7b0516007b99d58aee906eac62358625100ee32e

[dash] Don't pass args to subshell.

Change-Id: Ie62068048d58be46818c03cb428b68dd375594d6

[devmgr][mxio] Add a system-wide loader service

Add a dmctl ioctl to get a handle to a shared system loader service,
which translates paths like "libhid.so" into VMOs containing the named
object's data.

Launchpad and other users of mxio_loader_service() will automatically
prefer the system loader service if available.

Before this, loading was done by in-process threads, which meant that
those processes needed filesystem access. This is a step towards
supporting dynamic binaries with no filesystem rights.

For now, the system loader service uses the same code as the in-process
loader service, but this may change. It also needs to be smarter to
avoid starvation from bad processes (MG-491).

Moving the loader-related prototypes to a new header caused some ripple
effects for code that depended on transitive includes from mxio/util.h.

For MG-451: "loader service"

Change-Id: I8adb534b55f96334017e979da6af263e19095d84

[dash] Add pipe support.

Change-Id: Id2788359ddf4613eea2dc75f8a08448a64e51922

[dash] Add backtick support

Unify the process execution infrastructure and build support for
$(cmd) and `cmd` constructs.

Change-Id: I235a58fd09a258e7c8687d26b13b6cefc4ddedb1

[dash] Pass all variables to a subshell.

Change-Id: I6580cd0b957f0ab6c9348d6af94b3d359208a2f3

[launchpad] remove launchpad_get_mxio_job()

The general mx_job_default() supercedes this functionality.

Change-Id: I2758d4d533ef704f598de9b9f8118adfa538168e

[system][launchpad] Exposed cached job handle

From existing user and from trying to write tests
it is clear that we should give access to the cached
job handle (from mxio) that launchpad holds.

Else people are going to invent lesser versions for
example here dash.

Change-Id: I3ea287fe1abcb36bebd94db62ab60ff603420255

Pass the right environment to launchpad_environ()

Change-Id: I08ecd31cfde1a3937984199d288fca4ec936a6eb

[dash] Implement foreground subshells

Dash can now execute AST fragments in subprocesses, which means commands
like the following work:

$ (echo 42)
42

Change-Id: I66a994f9de9d3541d1c830c0db75ce09f3196e9f

[dash] Add support for ApplicationEnvironments

This feature is similar to the one in mxsh.

Change-Id: If19e1634132b4a557c5578c97f76e019bbd59883

[dash] Make basic commands work

After this patch, /bin/sh has a reasonable default path, can search it
and, actually run child processes.

Change-Id: I0ec5f8405226ec1254f9f772cbece5f116c28981