symlinkat: port to AIX 7.1

symlinkat: port to AIX 7.1

* doc/posix-functions/symlinkat.texi (symlinkat):
Mention AIX porting problem.
* lib/symlinkat.c: Always include errno.h.
(rpl_symlinkat) [HAVE_SYMLINKAT]: New function.
* lib/unistd.in.h (symlinkat): Add replacement machinery.
* m4/symlinkat.m4 (gl_FUNC_SYMLINKAT): Check symlinkat behavior.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add REPLACE_SYMLINKAT.
* modules/symlinkat (Depends-on): Add fstatat if REPLACE_SYMLINKAT.
(configure.ac): Also compile replacement if REPLACE_SYMLINKAT.
* modules/unistd (unistd.h): Substitute REPLACE_SYMLINKAT.