Add IPv6 support to the ng_ipfw(4) [1]. Also add ifdefs to be able
build it with and without INET/INET6 support.

Submitted by: Alexander V. Chernikov <melifaro at yandex-team.ru> [1]
Tested by: Alexander V. Chernikov <melifaro at yandex-team.ru> [1]
Approved by: re (bz)
MFC after: 2 weeks