fs-util: replace symlink_atomic_full_label() by a flag to symlinkat_atomic_full()

More porting work to label_ops_pre()/label_ops_post()

This also enables labelling of the /etc/localtime symlink in
systemd-firstboot, which should address one small facet of #37857

src/tmpfiles/tmpfiles.c

@@ -2471,9 +2471,7 @@ static int create_symlink(Context *c, Item *i) {
 
                 fd = safe_close(fd);
 
-                mac_selinux_create_file_prepare(i->path, S_IFLNK);
-                r = symlinkat_atomic_full(i->argument, pfd, bn, /* make_relative= */ false);
-                mac_selinux_create_file_clear();
+                r = symlinkat_atomic_full(i->argument, pfd, bn, SYMLINK_LABEL);
                 if (IN_SET(r, -EISDIR, -EEXIST, -ENOTEMPTY)) {
                         r = rm_rf_child(pfd, bn, REMOVE_PHYSICAL);
                         if (r < 0)