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

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

* m4/unlinkat.m4: New file, extracted from m4/openat.m4.
* m4/openat.m4 (gl_FUNC_OPENAT): Don't set GNULIB_UNLINKAT,
REPLACE_UNLINKAT, HAVE_UNLINKAT. Don't test for unlinkat.
* modules/unlinkat: New file, extracted from modules/openat. Correct
the dependency conditions.
* modules/openat (Files): Remove lib/unlinkat.c.
(Depends-on): Remove rmdir, unlink.
(configure.ac): Remove AC_LIBOBJ of unlinkat.
* modules/unlinkat-tests: New file, extracted from modules/openat-tests.
* modules/openat-tests (Files): Remove tests/test-unlinkat.c,
tests/test-rmdir.h, tests/test-unlink.h.
(Depends-on): Remove unlinkdir.
(Makefile.am): Remove rules for test-unlinkat.
* doc/posix-functions/unlinkat.texi: Mention module 'unlinkat' instead
of module 'openat'.
* NEWS: Mention the change.
* modules/linkat-tests (Depends-on): Add unlinkat.
* modules/mkfifoat-tests (Depends-on): Likewise.
* modules/readlinkat-tests (Depends-on): Likewise.