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

(cherry picked from commit a08cdb6cfb1c84b80b5337d46c574b55d0e15c63)