symlink: use throughout gnulib

symlink: use throughout gnulib

* m4/symlinkat.m4 (gl_FUNC_SYMLINKAT): Omit symlink check.
* lib/symlinkat.c (symlinkat) [!HAVE_SYMLINK]: Document why
symlink is not used.
* modules/symlinkat (Depends-on): Add symlink.
* modules/canonicalize-lgpl-tests (Depends-on): Likewise.
* modules/canonicalize-tests (Depends-on): Likewise.
* modules/lstat-tests (Depends-on): Likewise.
* modules/openat-tests (Depends-on): Likewise.
* modules/remove-tests (Depends-on): Likewise.
* modules/rmdir-tests (Depends-on): Likewise.
* modules/unlink-tests (Depends-on): Likewise.
* tests/test-canonicalize-lgpl.c (symlink): Delete stub.
* tests/test-canonicalize.c (symlink): Likewise.
* tests/test-fstatat.c (symlink): Likewise.
* tests/test-lstat.c (symlink): Likewise.
* tests/test-remove.c (symlink): Likewise.
* tests/test-rmdir.c (symlink): Likewise.
* tests/test-unlink.c (symlink): Likewise.
* tests/test-unlinkat.c (symlink): Likewise.

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