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/basic/process-util.c

@@ -807,7 +807,7 @@ int pid_from_same_root_fs(pid_t pid) {
 
         root = procfs_file_alloca(pid, "root");
 
-        return files_same(root, "/proc/1/root");
+        return files_same(root, "/proc/1/root", 0);
 }
 
 bool is_main_thread(void) {