netgraph/ng_nat: Add RFC 6598/Carrier Grade NAT support

This extends upon the RFC 6598 support to libalias/ipfw in r357092.

Reviewed By: manpages (bcr), donner, adrian, kp
Approved by: kp (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23461