mkfifoat: Work around trailing slash bug in mknodat() on AIX 7.2.
* m4/mkfifoat.m4 (gl_FUNC_MKFIFOAT): Set also REPLACE_MKNODAT.
* lib/sys_stat.in.h (mknodat): Consider REPLACE_MKNODAT.
* lib/mknodat.c: Add an overriding implementation of mknodat().
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Initialize REPLACE_MKNODAT.
* modules/sys_stat (Makefile.am): Substitute REPLACE_MKNODAT.
* modules/mkfifoat (Depends-on, configure.ac): Consider REPLACE_MKNODAT.
* doc/posix-functions/mknodat.texi: Mention the AIX bug.