openat: test for fstatat (..., 0) bug

openat: test for fstatat (..., 0) bug

Further testing with tar suggests that fstatat (..., 0)
does not work in general, on AIX 7.1; see
<http://lists.gnu.org/archive/html/bug-tar/2011-09/msg00023.html>.
So, give up entirely on AIX 7.1's fstatat, and fall back on our
replacement fstatat (which is what older AIX releases were using
anyway).
* lib/fstatat.c (fstatat) [HAVE_FSTATAT]: Do not undef.  The only
use is now changed to orig_fstatat.  This was probably the right
thing to do anyway.
(FSTATAT_AT_FDCWD_0_BROKEN): Remove; no longer used.
(rpl_fstatat) [FSTATAT_ZERO_FLAG_BROKEN]: Remove.
(rpl_fstatat): Simplify, assuming !FSTATAT_ZERO_FLAG_BROKEN.
(AT_FUNC_NAME) [FSTATAT_ZERO_FLAG_BROKEN]: Now rpl_fstatat.
* m4/openat.m4 (gl_FUNC_FSTATAT): Test for the more-general bug
and define FSTATAT_ZERO_FLAG_BROKEN, not FSTATAT_AT_FDCWD_0_BROKEN,
if the bug is found.