[dash] fix validation of nodes length

Previously the condition asserted that the buffer to decode was exactly
the declared length. This was more aggressive than needed, especially as
there are valid reasons for the buffer being larger, such as padding.
The new condition asserts that the buffer is at least as large as its
declared length.

ZX-1920 #fix

Change-Id: I7cbd0c5a6782dce4872bcf66c03b841124999d33

[syscall][vmo] removal of zx_vmo_{read,write}() actual parameter 4/n

This reverts commit d7c91f0a8a876bd4de8808595cafcd4f2c3fd7a1.

Change-Id: I5c61410f62850583a970d3ff2f44b6d58901509a

[revert] Revert "[syscall][vmo] removal of zx_vmo_{read,write}() actual parameter 4/n"

This reverts commit 2d25909b7ab244d73172c9f5bc11d85b49ccefbb.

Change-Id: I09efe4e1d69d7aeadb051936d3ddc8237e7d39cf

[syscall][vmo] removal of zx_vmo_{read,write}() actual parameter 4/n

Remove actual from zx_vmo_{read,write}
Mark zx_vmo_{read,write}_old as deprecated
Retarget most uses of zx_vmo_{read,write}_old to zx_vmo_{read,write}.

Still need to migrate a handful of uses of _old that had meaningful
usage of actual.
This is good enough to go ahead and migrate uses in Garnet and Topaz.

ZX-1802

Change-Id: If57329f9c5183a0dacd29083decbaf771925145f

[dash] Delete unused variable

Change-Id: If4f4782b7038d7dab0bc5b34438422dfe2efec25

[dash] Validate nodes length

Change-Id: I917998d77be2cd8a80970dd9838b7f6b32ddfed1

[syscall][vmo] removal of zx_vmo_{read,write}() actual parameter 1/n

In preparation for changing the zx_vmo_read() and write() syscalls,
introduce aliases (zx_vmo_read_old() and write_old()) for the existing
behaviour, migrate zircon callers to the aliases, and mark the original
syscalls as deprecated.

Once the garnet and above layers have been migrated, the original
syscalls will be adjusted to remove the "actual" argument, and the
temporary _old() versions will be deprecated, and then removed.

ZX-1802

Change-Id: I2fab9a59caf4093fb585ada6015197017767aba2

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[dash] Migrate to the new MX_ error codes

Change-Id: I82e4ba00bc1cbe3609f74781b505d6476a7b2365

[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] 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

[dash] Add linenoise tab completion to dash

Change-Id: I6731af42100aed4a02080cbf8a6caa3c8b20afd7

Remove mx_size_t and mx_ssize_t types

mx_ssize_t is no longer used. mx_size_t is superfluous and
can be replaced by the standard C type size_t.

Change-Id: I2137af6f166099d2e8aaca4fab63ff101f2a43ac

[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 dash to the build

This patch integrates dash with the build system, including
pre-generating source files.

Change-Id: If19d0be0c030c6c194cb7bf66a26be48a39c0847