Per CMAKE docs, CMAKE_HOST_SYSTEM_VERSION is the result of `uname -r`:
https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_SYSTEM_VERSION.html?highlight=uname

    A numeric version string for the system. On systems that support
    uname, this variable is set to the output of uname -r. On other
    systems this is set to major-minor version numbers.

On Windows it is something like "6.1", so it won't match ".*-Microsoft".

Closes #7329

New logging is guarded by cmake LOG_LIST_ACTIONS define. To make it more
efficient it is allocated as a linked list with chunks of length
2^(7+chunk_num); that uses basically the same idea as behind increasing kvec
length (make appending O(1) (amortized)), but reduces constant by not bothering
to move memory around what realloc() would surely do: it is not like we need
random access to log entries here to justify usage of a single continuous memory
block.

Closes #7099

Requires libtermkey 0.19+

Closes #2048
Closes #5693

See https://github.com/neovim/libtermkey/compare/a9b61424aae9f7548162ff112393c5f706cf54f1%5E...c0eb4e4a05f49ad8fee0195c77f2c29d09cc36af
See https://bugzilla.redhat.com/show_bug.cgi?id=142659
See https://github.com/tmux/tmux/blob/fe4e9470bb504357d073320f5d305b22663ee3fd/tty-keys.c#L625-L632

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.