statat: new module, split out from fstatat

statat: new module, split out from fstatat

GNU Emacs needs the POSIX-specified fstatat, but not the
gnulib-specified statat and lstat.  Split the latter two into a
new module 'statat'.
* lib/openat.h: Depend on GNULIB_STATAT, not GNULIB_FSTATAT.
* lib/openat.h, lib/statat.c (STATAT_INLINE):
Rename from FSTATAT_INLINE. All uses changed.
* modules/fstatat (Files): Remove lib/statat.c.
(gl_MODULE_INDICATOR([fstatat])): Remove.
(lib_SOURCES): Remove.
(Maintainer): Add self.
* modules/statat, modules/statat-tests, tests/test-statat.c: New files.
* tests/test-fstatat.c (BASE): Don't define if already defined.
(do_stat, do_lstat) [!TEST_STATAT]: Test fstatat instead.