helpers: Update license to dual-licensed GPL and BSD

We want to encourage reuse of the helper functions, so add a BSD licensing
option as well. Also, update the GPL specification to 2.0-or-later, since
the GPL-2.0 SPDX tag is deprecated, and with the BSD option, licensing as
2.0-only doesn't really make sense, since the BSD license permits inclusion
in GPL-3 works anyway.

All copyright holders of the affected files have signed off on this change,
as per the discussion in #80.

Fixes #80.

Signed-off-by: Toke Høiland-Jørgensen <[email protected]>

Author: tohojo

common/parsing_helpers.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-clause) */
 /*
  * This file contains parsing functions that are used in the packetXX XDP
  * programs. The functions are marked as __always_inline, and fully defined in

common/rewrite_helpers.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-clause) */
 /*
  * This file contains functions that are used in the packetXX XDP programs to
  * manipulate on packets data. The functions are marked as __always_inline, and