Add application rules for the ufw firewall

For Avahi to work when a firewall is in use, you need to allow udp traffic to port 5353. The corresponding iptables command is:

iptables -A INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT
iptables -A OUTPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT

This is obviously hard to figure out for non-technical users, the type of user that would benefit most of Avahi "just working."

To simplify this, Avahi should ship with a ufw profile. See https://wiki.ubuntu.com/UbuntuFirewall#Package%20Integration