Fix/Update copyright notices; Add SPDX annotation

Let's fix some erroneous/accidential references to wrong license,
update copyright information where applicable and introduce a
SPDX-License-Identifier to all files.

Change-Id: I39af26c6aaaf5c926966391f6565fc5936be21af
diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index 8daf460..5cfe1ec 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -16,10 +16,12 @@
  *  \file gsup.h
  *  Osmocom Generic Subscriber Update Protocol message encoder/decoder. */
 /*
- * (C) 2014 by sysmocom s.f.m.c. GmbH, Author: Jacob Erlbeck
+ * (C) 2014 by sysmocom - s.f.m.c. GmbH, Author: Jacob Erlbeck
  * (C) 2016 by Harald Welte <laforge@gnumonks.org>
  * All Rights Reserved
  *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; either version 2 of the License, or