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.

We do not use it and all systems we support have <fcntl.h>.

Although UNIX is a registered trademark of The Open Group, it doesn't
really matter whether we refer to these systems as UNIX, Unix, or
Unix-like. So, for consistency, refer to them collectively as Unix.

Related:
http://www.greens.org/about/unix.html
http://www.unixica.com/html/unixunix.html

HAVE_CRT_EXTERNS_H is unused since a3ec68c.
SIZEOF_OFF_T is unused since f916cf0.
HAVE_LIBGEN_H was never used.

In Vim, at least the constant `I_PUSH` is used from <stropts.h>, but
Neovim doesn't seem to use anything from said header.

Besides that, POSIX.1-2008[1] marks this header as obsolescent, and
there don't seem to be many platforms that even have it.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stropts.h.html#tag_13_52_11

For non-Windows systems, we assume that NAME_MAX is in <limits.h>, as
specified in POSIX.1-2008[1]. For Windows, which doesn't have NAME_MAX,
just define it ourselves to _MAX_PATH[2].

Also, remove two (now unused) HAVE_*_H checks.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html
[2]: https://msdn.microsoft.com/en-us/library/930f87yf.aspx

Helped-by: Seth Jackson

Remove "Commit:" field from `--version` output

Windows does not have setenv(), instead the _putenv_s() function is used - added
a function check and fatal errors. Implemented os_setenv() for Windows.

Vim supports the original putenv() function if no alternative is available.
Neovim only supports systems where safer alternatives exist, so the check for
putenv() was removed from config/CMakeLists.txt.