Overhaul scaninc to work recursively

This also fixes the bug where scaninc would ignore #include lines
in assembly files.

src/field_message_box.c

@@ -1,6 +1,6 @@
+#include <string.h>
 #include "global.h"
 #include "menu.h"
-#include "string.h"
 #include "string_util.h"
 #include "task.h"
 #include "text.h"