rc.d: Add ipfw3 rc script

rc.d: Add ipfw3 rc script

To use ipfw3, add 'ipfw3_enable=YES' to /etc/rc.conf .  By default, only
the 'ipfw3' and 'ipfw3_basic' modules will be loaded.  If you need more
ipfw3 modules, set them with 'ipfw3_modules="<modules> ..."' in
/etc/rc.conf .  See also the rc.conf(5) man page.

This ipfw3 rc script is largely based on the ipfw script.

The /etc/ipfw3.rules (a shell script) should be present to setup the
firewall rules, otherwise the default setup will block all incoming
connections, including your current connection!

NOTE:
I wrote this script for use in my VPS last year, but switched to
PF later, so more tests and improvements (e.g., flush tables on stop)
to this script are needed.

Reviewed-by: swildner