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/src/stats.c b/src/stats.c
index ff31ff6..b5adbf2 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -1,11 +1,11 @@
 /*! \file stats.c */
 /*
- * (C) 2015 by Sysmocom s.f.m.c. GmbH
- *
+ * (C) 2015 by sysmocom - s.f.m.c. GmbH
  * Author: Jacob Erlbeck <jerlbeck@sysmocom.de>
- *
  * 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