miniupnpd/pcpserver.c: port NAT-PMP updates to PCP

This change ports the recent updates to the permissions checking
and eport selection code for NAT-PMP to the PCP MAP handler.
Merge pull request #59 from razzfazz/port_natpmp_updates_to_pcp

miniupnpd/pcpserver.c: port NAT-PMP updates to PCP