Allow setting alias port ranges in libalias and ipfw. This will allow a system
to be a true RFC 6598 NAT444 setup, where each network segment (e.g. user,
subnet) can have their own dedicated port aliasing ranges.

Reviewed by: donner, kp
Approved by: 0mp (mentor), donner, kp
Differential Revision: https://reviews.freebsd.org/D23450