fff-gateway: Add firewall rules to ensure nothing is forwarded onto WAN

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>

src/packages/fff/fff-gateway/files/usr/lib/firewall.d/10-no-forward-wan

+# Ensure nothing is forwarded onto WAN interface
+iptables -A FORWARD -o $IF_WAN -j REJECT --reject-with icmp-net-unreachable
+ip6tables -A FORWARD -o $IF_WAN -j REJECT --reject-with no-route