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
Differential Revision: https://reviews.freebsd.org/D23461
(cherry picked from commit 5fe433a6e4d8cab6b64284698301afc0c55a9db2)