Add FullconeNAT Support

Created by: hikaruchang

Patch by Cion82(https://github.com/Chion82/netfilter-full-cone-nat)

mplementation of RFC3489-compatible full cone SNAT.

Assuming eth0 is external interface:

iptables -t nat -A POSTROUTING -o eth0 -j FULLCONENAT #same as MASQUERADE  
iptables -t nat -A PREROUTING -i eth0 -j FULLCONENAT  #automatically restore NAT for inbound packets

Currently only UDP traffic is supported for full-cone NAT. For other protos FULLCONENAT is equivalent to MASQUERADE.