[doc] Give files a common structure by placing license first (partial)

Followed by doxyen file and group tags.
This patch is only a first attempt to make provide a clean structure,
many more files require rework, too.

apps/servreg-hack/servreg-hack.c

@@ -1,6 +1,3 @@
-/** \addtogroup servreghack
- * @{ */
-
 /*
  * Copyright (c) 2010, Swedish Institute of Computer Science.
  * All rights reserved.
@@ -40,6 +37,9 @@
  *         Adam Dunkels <adam@sics.se>
  */
 
+/** \addtogroup servreghack
+ * @{ */
+
 #include "contiki.h"
 #include "contiki-lib.h"
 #include "contiki-net.h"