Improved interface handling. This fixes the use of renamed interfaces
(ifconfig xl0 name foo) as well as some special interfaces such as the
6to4 tunnel.

Reported by:    Ed Schouten <ed (at) il ! fontys , nl>
Tested by:      freebsd-pf
PR:             kern/[[ https://bugs.freebsd.org/72444 | 72444 ]]