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/vty/vty.c b/src/vty/vty.c
index aef73b3..e741a66 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -22,7 +22,7 @@
  *
  * \section sec_copyright Copyright and License
  * Copyright © 1997-2007 - Kuninhiro Ishiguro\n
- * Copyright © 2008-2011 - Harald Welte, Holger Freyther and contributors\n
+ * Copyright © 2008-2012 - Harald Welte, Holger Freyther and contributors\n
  * All rights reserved. \n\n
  * The source code of libosmovty is licensed under the terms of the GNU
  * General Public License as published by the Free Software Foundation;
@@ -46,6 +46,7 @@
  * <http://sysmocom.de/>
  */
 
+/* SPDX-License-Identifier: GPL-2.0+ */
 
 #include <stdio.h>
 #include <stdarg.h>