unlink: new module, for Solaris 9 bug

unlink: new module, for Solaris 9 bug

unlink("file/") mistakenly succeeded.  This patch favors, but
does not enforce, GNU semantics that unlink("link-to-dir/")
flat-out fails rather than attempting to unlink "dir".

* modules/unlink: New file.
* lib/unlink.c: Likewise.
* m4/unlink.m4 (gl_FUNC_UNLINK): Likewise.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witnesses.
* modules/unistd (Makefile.am): Use them.
* lib/unistd.in.h (stat): Declare replacement.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/unlink.texi (unlink): Likewise.
* modules/unlink-tests: New test.
* tests/test-unlink.c: Likewise.

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