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

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

* lib/openat.h (chmodat, lchmodat): Enable only if GNULIB_FCHMODAT is
defined.
* m4/fchmodat.m4: New file, extracted from m4/openat.m4.
* m4/openat.m4 (gl_FUNC_OPENAT): Don't set GNULIB_FCHMODAT. Don't test
for fchmodat, lchmod. Don't set HAVE_FCHMODAT.
* modules/fchmodat: New file, extracted from modules/openat.
* modules/openat (Files): Remove lib/fchmodat.c.
(configure.ac): Remove AC_LIBOBJ of fchmodat.
* modules/fchmodat-tests: New file, extracted from modules/openat-tests.
* modules/openat-tests (Files): Remove tests/test-fchmodat.c.
(Makefile.am): Remove rules for test-fchmodat.
* doc/posix-functions/fchmodat.texi: Mention module 'fchmodat' instead
of module 'openat'.
* NEWS: Mention the change.