* Add What-Is-Network-Number handling.
Add What-Is-Network-Number and Network-Number-Is network layer handling.
Refactor npdu_encode_npdu_network() from router specific code.
Add unit test for NDPU network message
Add app for What-Is-Network-Number
Add app for Network-Number-Is
Add send helper for What-Is-Network-Number
Add send helper for Network-Number-Is
* added sys/debug.c to ports builds for use of debug_printf() in npdu handler.
Co-authored-by: Steve Karg <skarg@users.sourceforge.net>
Steve Karg