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/config.c

@@ -1,3 +1,7 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2011-2013, Christopher Jeffrey
+// Copyright (c) 2013 Richard Grenville <pyxlcy@gmail.com>
+
 #include <stdlib.h>
 #include <stdbool.h>