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

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

* m4/mkdirat.m4: New file. extracted from m4/openat.m4.
* m4/openat.m4 (gl_FUNC_OPENAT): Don't require gl_SYS_STAT_H_DEFAULTS.
Don't test for mkdirat. Don't set GNULIB_MKDIRAT, HAVE_MKDIRAT.
* modules/mkdirat: New file, extracted from modules/openat.
* modules/openat (Files): Remove lib/mkdirat.c.
(Depends-on): Remove mkdir.
(configure.ac): Remove AC_LIBOBJ of mkdirat.
(Include): Remove <sys/stat.h>.
* modules/mkdirat-tests: New file, extracted from modules/openat-tests.
* modules/openat-tests (Files): Remove tests/test-mkdirat.c,
tests/test-mkdir.h.
(Depends-on): Remove ignore-value.
(Makefile.am): Remove rules for test-mkdirat.
* doc/posix-functions/mkdirat.texi: Mention module 'mkdirat' instead
of module 'openat'.
* NEWS: Mention the change.