mkfifoat: Work around trailing slash bug in mkfifoat() on AIX 7.2.
* m4/mkfifoat.m4 (gl_FUNC_MKFIFOAT): Add a test whether mkfifoat rejects
trailing slashes. Set REPLACE_MKFIFOAT if not.
* lib/sys_stat.in.h (mkfifoat): Consider REPLACE_MKFIFOAT.
* lib/mkfifoat.c: Add an overriding implementation of mkfifoat().
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Initialize
REPLACE_MKFIFOAT.
* modules/sys_stat (Makefile.am): Substitute REPLACE_MKFIFOAT.
* modules/mkfifoat (Depends-on): Add fstatat.
(configure.ac): Consider REPLACE_MKFIFOAT.
* doc/posix-functions/mkfifoat.texi: Mention the AIX bug.