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/xrescheck.c

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2014 Richard Grenville <pyxlcy@gmail.com>
+
 #include "xrescheck.h"
 
 static xrc_xid_record_t *gs_xid_records = NULL;