Sort out license problems

IANAL, but I think I am allowed to add missing copyright notices for
someone else. And I did my best job using git history to figure out who
wrote which functions. So I hope everything is fine.

src/utils.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2018 Yuxuan Shui <yshuiv7@gmail.com>
 #pragma once
 #include <ctype.h>
 #include <stddef.h>