openat: work around AIX 7.1 fstatat issue

This should fix the problem that was not properly fixed
in the previous change, dated 2011-08-30.
* lib/fstatat.c: Include <sys/stat.h> twice, the first with
__need_system_stat_h defined.
(orig_fstatat) [HAVE_FSTATAT]: New function.
(rpl_fstatat): Go back to the old way of doing things,
except call orig_fstatat instead of fstatat.
* m4/openat.m4 (gl_FUNC_FSTATAT): Remove unnecessary check for openat.
Remove unnecessary check whether fstatat fills in st_size etc.
(cherry picked from commit de1fa2ac9aeb9f70a042ee3faa2b7712e65a278b)