firewall: add rule for traceroute support

firewall: add rule for traceroute support

Running your firewall's "wan" zone in REJECT zone (1) exposes the
presence of the router, (2) depending on the sophistication of
fingerprinting tools might identify the OS and release running on
the firewall which then identifies known vulnerabilities with it
and (3) perhaps most importantly of all, your firewall can be
used in a DDoS reflection attack with spoofed traffic generating
ICMP Unreachables or TCP RST's to overwhelm a victim or saturate
his link.

This rule, when enabled, allows traceroute to work even when the
default input policy of the firewall for the wan zone has been
set to DROP.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>