API level is disconnected from NVIM version. The API metadata holds the
current API level, and the lowest backwards-compatible level supported
by this instance.

Release 0.1.6 is the first release that reports the Nvim version and API
level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      api_level: 1,
      api_compatible: 0,
      api_prerelease: false,
    }

The API level may remain unchanged across Nvim releases if the API has
not changed.

When changing the API,
    - set NVIM_API_PRERELEASE to true
    - increment NVIM_API_LEVEL (at most once per Nvim version)
    - adjust NVIM_API_LEVEL_COMPAT if backwards-compatibility was broken

api_level_0.mpack was generated from Nvim 0.1.5 with:
    nvim --api-info

The API level is disconnected from the NVIM version. The API metadata
holds the current API level, and the lowest backwards-compatible level
supported by this instance.

Release 0.1.6 will be the first release reporting the Nvim version and
API level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      prerelease: true,
      api_level: 1,
      api_compatible: 0,
    }

The API level may remain unchanged across Neovim releases if the API has
not changed.

When changing the API the CMake variable NVIM_API_PRERELEASE is set to
true, and  NVIM_API_CURRENT/NVIM_API_COMPATIBILITY are incremented
accordingly.

The functional tests check the API table against fixtures of past
versions of Neovim. It compares all the functions in the old table with
the new one, it does ignore some metadata attributes that do not alter
the function signature or were removed since 0.1.5.  Currently the only
fixture is 0.mpack, generated from Neovim 0.1.5 with nvim --api-info.

Allowing this to be controlled externally improves reproducibility, as
well as provides a more useful address to report for "Compiled by".  For
example, I intend to set it to the packaging list when building the
Debian package.

Signed-off-by: James McCoy <jamessan@jamessan.com>

Still no busted tests. Not tested without HAVE_PREADV.

Moves low-level functions handling to os/fs.c. Adds file.c with a proxy
interface.

Target: while leaving syscalls handling is os.c (partially handled by libuv),
add buffering for reading and writing to file.c.

The TUI can be enabled/disabled at build time with -DFEAT_TUI, default is ON for
UNIX, and OFF for non UNIX. When off, Neovim prints a message to stderr, along
with a list of the server endpoints.

This was noticed due to a user issue (#4750) when building Neovim 0.1.4
via ports.  The crux of the issue is that we did not detect the
be64toh() macro, because there is no endian.h on FreeBSD (along with
several other BSDs).  So we were defaulting to our builtin version of
be64toh().  However, it appears that sys/endian.h was being picked up by
an include (likely msgpack.h) and so be64toh() was actually defined and
corrupting our definition of it.

So the answer here was to use the correct include file in our check, and
export that information in the config.h.  Then we use that information
to include the right header in shada.c.

This fixes #4750.

Change POROJECT_NAME to 'nvim', and use it as the gettext
domain name. The *.mo files, previously installed as
$runtime/lang/xx/LC_MESSAGES/nvim.mo, are now installed as
$prefix/locale/xx/LC_MESSAGES/nvim.mo.

Unix systems must have this header but Windows does not have it at all.

Since src/nvim/os/unix_defs.h includes <unistd.h> without the guard
in order to avoid including this in the numerous places we would
need <unistd.h> on Unix we just include src/nvim/os/os.h which will pull
in <unistd.h> for us.