Allow a compile-time override of the ipfw deny rule. For a 'firewall'
you don't want this (and the documentation explains why), but if you
use ipfw as an as-needed casual filter as needed which normally runs as
'allow all' then having the kernel and /sbin/ipfw get out of sync is a
*MAJOR* pain in the behind.

PR: 4141
Submitted by: Heikki Suonsivu <hsu@mail.clinet.fi>