Add support for RFC 6598/Carrier Grade NAT address ranges (100.64.0.0/10) in natd by adding a natd flag -unregistered_cgn and its short version -c. This is similar to the -unregistered_only option for RFC 1918, however those subnets will also be supported by this new option.
This extends upon r357092 where we added support for RFC 6598 subnets in libalias and ipfw.
Submitted by: Neel Chauhan <neel AT neelc DOT org>