UPnP is now used to request a port on the NAT be forwarded to the local machine.
This obviously only works for NATs that support UPnP, and thus is not a panacea
for all NAT-related connectivity issues.
Notable hacks in this patch include a transmit/receive UDP socket pair to work
around current protocol limitations whereby the full node assumes its peer can
receive on the same UDP port it transmitted from.
