packet.lua: Fix broken clone().
main.exit(): Fixed bug - always used exit status 0.
socket/dev.lua: Updated after packet.lua API change.
ns_responder: Make north->south direction optional.

Use packet.want_modify().
packet.h: buffer data is now 'unsigned char *' (was char *)
Added apps/l2tpv3.

This is an additional L2TPv3 app created for quick interop testing
with a Cisco ASR9000.
Added l2tpv3 design.