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

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

This tests for another fstatat bug on AIX 7.1:
fstatat (AT_FDCWD, ..., 0) does not work.  See
<http://lists.gnu.org/archive/html/bug-tar/2011-09/msg00015.html>.
* lib/fstatat.c (FSTATAT_AT_FDCWD_0_BROKEN)
(LSTAT_FOLLOWS_SLASHED_SYMLINK): Default to 0.
(rpl_fstatat): Adjust so that it works around either (or both)
bugs if present.
* m4/openat.m4 (gl_FUNC_FSTATAT): Test for this fstatat bug.
(cherry picked from commit e91216b238f2af78257bd943db0b22b289d172cc)