basic/path-util: allow flags for path_equal_or_files_same

No functional change, just a new parameters and the tests that
AT_SYMLINK_NOFOLLOW works as expected.

src/systemctl/systemctl.c

@@ -5654,7 +5654,7 @@ static int switch_root(int argc, char *argv[], void *userdata) {
 
                 /* If the passed init is actually the same as the
                  * systemd binary, then let's suppress it. */
-                if (files_same(root_init_path, root_systemd_path) > 0)
+                if (files_same(root_init_path, root_systemd_path, 0) > 0)
                         init = NULL;
         }