getcwd: break fdopendir + save_cwd recursive loop (Bug#13516)

getcwd: break fdopendir + save_cwd recursive loop (Bug#13516)

Reported for OS X 10.8.2 by Assaf Gordon in
<http://bugs.gnu.org/13516>.
* lib/getcwd.c (HAVE_OPENAT_SUPPORT): Do not define if
!HAVE_OPENAT && !HAVE_FDOPENDIR.
* m4/getcwd-abort-bug.m4: Reformat to match test-getcwd.c
so that they can be kept in sync more easily.  Avoid PATH_MAX
test on the Hurd.  Sync from test-getcwd.c for errno tests after
mkdir or chdir failure.
* tests/test-getcwd.c (HAVE_OPENAT_SUPPORT): New macro, from
lib/getcwd.c.
(test_abort_bug): Do not test for the deep directory bug unless we
have openat support.  Avoid PATH_MAX test on the Hurd.