ipfw: add ability to run ipfw(8) binary with 15.0+ kernel module

After D46183 the KBI was changed and this made the upgrade procedure
to 15.0+ version a bit difficult, because the old binary can not load
firewall rules when the new kernel is loaded.

This commit adds the sbin/ipfw15 binary that uses new KBI, and then
original sbin/ipfw can detect new KBI and run the new binary instead.

Approved by: re (cperciva)
PR: 291562
Reviewed by: jhb, glebius
Fixes: 4a77657cbc01
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54763

(cherry picked from commit 704ec5e68c44f08d83f3b0daa315c6143338863f)