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.