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/telnet_interface.c b/src/vty/telnet_interface.c
index 40154fe..0ccf8dc 100644
--- a/src/vty/telnet_interface.c
+++ b/src/vty/telnet_interface.c
@@ -1,6 +1,9 @@
-/* (C) 2009 by Holger Hans Peter Freyther <zecke@selfish.org>
+/* (C) 2009-2010 by Holger Hans Peter Freyther <zecke@selfish.org>
+ * (C) 2010 by Sylvain Munaut <tnt@246tNt.com>
  * 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