New module 'fchownat', split off from module 'openat'.

New module 'fchownat', split off from module 'openat'.

* lib/openat.h (chownat, lchownat): Enable only if GNULIB_FCHOWNAT is
defined.
* m4/fchownat.m4: New file, extracted from m4/openat.m4.
* m4/openat.m4 (gl_FUNC_OPENAT): Don't set GNULIB_FCHOWNAT. Don't
invoke gl_FUNC_FCHOWNAT.
(gl_FUNC_FCHOWNAT_DEREF_BUG, gl_FUNC_FCHOWNAT_EMPTY_FILENAME_BUG,
gl_FUNC_FCHOWNAT): Moved to m4/fchownat.m4.
* modules/fchownat: New file, extracted from modules/openat.
* modules/openat (Files): Remove lib/fchownat.c.
(Depends-on): Remove lchown.
(configure.ac): Remove AC_LIBOBJ of fchownat.
* modules/fchownat-tests: New file, extracted from modules/openat-tests.
* modules/openat-tests (Files): Remove tests/test-fchownat.c,
tests/test-chown.h, tests/test-lchown.h, tests/nap.h.
(Depends-on): Remove mgetgroups, usleep, stat-time.
(configure.ac): Remove test for getegid.
(Makefile.am): Remove rules for test-fchownat.
* doc/posix-functions/fchownat.texi: Mention module 'fchownat' instead
of module 'openat'.
* NEWS: Mention the change.