openat: fix unlinkat bugs on Solaris 9

openat: fix unlinkat bugs on Solaris 9

unlinkat(fd,"file/",0) mistakenly succeeded.

* lib/unlinkat.c (unlinkat): New file.
* modules/openat (Depends-on): Add unlink.
(Files): Distribute it.
* m4/openat.m4 (gl_FUNC_OPENAT): Mark unlinkat for replacement if
trailing slash behavior is broken.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Substitute it.
* lib/unistd.in.h (unlinkat): Declare replacement.
* doc/posix-functions/unlinkat.texi (unlinkat): Document this.

Signed-off-by: Eric Blake <ebb9@byu.net>