Add support for RFC 6598/Carrier Grade NAT subnets. to libalias and ipfw.

In libalias, a new flag PKT_ALIAS_UNREGISTERED_RFC6598 is added.
This is like PKT_ALIAS_UNREGISTERED_ONLY, but also is RFC 6598 aware.
Also, we add a new NAT option to ipfw called unreg_cgn, which is like
unreg_only, but also is RFC 6598-aware. The reason for the new
flags/options is to avoid breaking existing networks, especially those
which rely on RFC 6598 as an external address.

Submitted by: Neel Chauhan <neel AT neelc DOT org>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22877