[async] Tidy up the libasync API for tasks and receivers.

Remove flags fields and reserved storage.
Make the C++ wrappers friendlier.
Move the ops invokers into libasync.a itself in preparation for
adding versioning to the ops structure.
Add *_or_report_error() function variants which make it easier
to systematically handle errors while posting tasks or
enqueuing packets.
Removed the return value from task handlers to reduce confusion
around the lifetime of the task structure and remove an
unusual error handling scenario in case of a failure to
schedule a task to be repeated.
Pass the Task* to the handler for consistency with the C API.
Added some missing static library dependencies.
Reduce use of libfbl (except fbl/function.h).
Remove uses of private inheritance.

Change-Id: I7ccea5b68d6bccb2c7a236d112428c9b17fe8cdd

[async] Move async-loop implementation to its own directory.

People keep getting confused by the relationship between async-loop
and the rest of the library. So let's fix it.

This change includes compatibility includes for loop.h to be removed
after upper layers are fixed to use the new paths.

Change-Id: I95c57be533fbd91468275bb2470890a4f10101eb

[loader-service] Remove simple loader service

The "simple" version was used only for tests. Now we test the real code.
Removing the simple loader lets us simplify the real loader a bit.

Change-Id: I760fd06f430d156614f66c880d865882e8f286c8

[loader-service] Separate loader-service from launchpad

There's no particular reason to link the code for implementing the
shared library service into the library for launching executables.

This CL also removes the unused local shared library loader codepath,
which would secretly spin up a thread in your process to export your
namespace to processes you created if your parent didn't provide you
with a functional shared library loader service.

Change-Id: I0f2a31197df49831a7630ff72cf8a6a561c3dbd0

[ldmsg] Convert the loader service to FIDL wire format

Previously, the loader service used a hand-rolled protocol. Now the
protocol is specified in //zircon/system/public/zircon/loader.fidl.

Because this protocol is used by such low-level components (e.g.,
userboot and libc), we do not use generated code to implement the
protocol. Instead, we use a stand-alone library, ldmsg, which manually
encodes and decodes messages according to the wire format specified in
loader.fidl.

This CL also moves fidl/types.h into zircon/fidl.h because these type
definitions are now fundamental to the operation of the system. Putting
their definition in this location makes them easier to reference from
userboot and libc.

Change-Id: I9a7619b09b0ff543b8883ac53852ade9d5c033b6

Revert "[ldmsg] Convert the loader service to FIDL wire format"

This reverts commit 3ff2a3f316904d6acf709169fb12ee059231d5d9.

Additional review comments need to be addressed.

Original change's description:
> [ldmsg] Convert the loader service to FIDL wire format
>
> Previously, the loader service used a hand-rolled protocol. Now the
> protocol is specified in //zircon/system/public/zircon/loader.fidl.
>
> Because this protocol is used by such low-level components (e.g.,
> userboot and libc), we do not use generated code to implement the
> protocol. Instead, we use a stand-alone library, ldmsg, which manually
> encodes and decodes messages according to the wire format specified in
> loader.fidl.
>
> This CL also moves fidl/types.h into zircon/fidl.h because these type
> definitions are now fundamental to the operation of the system. Putting
> their definition in this location makes them easier to reference from
> userboot and libc.
>
> Change-Id: I806455813d3a397a2eff41badadf61c28812e328

TBR=kulakowski@google.com,swetland@google.com,abarth@google.com

Change-Id: I7ac90b53cdc152dc8cd892eb394ec7f641161ee9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true

[ldmsg] Convert the loader service to FIDL wire format

Previously, the loader service used a hand-rolled protocol. Now the
protocol is specified in //zircon/system/public/zircon/loader.fidl.

Because this protocol is used by such low-level components (e.g.,
userboot and libc), we do not use generated code to implement the
protocol. Instead, we use a stand-alone library, ldmsg, which manually
encodes and decodes messages according to the wire format specified in
loader.fidl.

This CL also moves fidl/types.h into zircon/fidl.h because these type
definitions are now fundamental to the operation of the system. Putting
their definition in this location makes them easier to reference from
userboot and libc.

Change-Id: I806455813d3a397a2eff41badadf61c28812e328

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[build][musl] refer to musl libc as ulib/c

This makes the build rules and the intermediate
output files easier to understand, without having
to modify the path where musl libc lives.

The biggest observeable difference is the build
intermediate files:

$ ls build-magenta-pc-x86-64/ulib/c
config-module.h libc.abi.h libc.abi.stamp libc.so libc.so.id libc.so.strip
libc.a libc.abi.o libc.a.opts libc.so.abi libc.so.opts third_party

which previously were at build-magenta-pc-x86/ulib/musl
and named libmusl.*

and the replacing of ulib/musl with ulib/c in the library
dependencies of all the rules.mk files (the vast bulk of
this change)

Change-Id: If9c5d522099d2c9ac5bdb666b8e67a2eb3d316ee

[system] remove unneeded deps on libruntime

Change-Id: Ia612d777b36dada7fda112aa41a6d7a49a212da0

[system] align license and license banners with Fuchsia

This adjusts the Magenta userspace code to match the prevailing
license in Fuchsia (three-clause BSD), using the same banner
and license text.

Change-Id: I6b6d79435c5dbaa32027888874849e4f47aaa7be

[ulib] [launchpad] Use mxr_mutex_t instead of C11 mtx_t

The low-level libraries like launchpad can't assume that their
callers will be using full-fledged C11 or POSIX threads. So don't
use standard synchronization methods, but only the ulib/runtime
layer instead.

Change-Id: Id861e56f78667249ec24910c9fc04514273bfe0b

[ulib] [musl] Add dlopen_vmo function

This adds a Magenta-specific extension to -ldl, declared in the
new header file <magenta/dlfcn.h>. The new function behaves like
dlopen, but takes a VM object handle instead of a file name.

Change-Id: Ia0748cf7b3e4c40ae0bf563f6d9f3182da04547b