Added PCP third party option and made it's use an option.
miniupnpd: change "allow_thirdparty" PCP option to a bit flag
miniupnpd/pcpserver.c: Some IPv6 related work on PCP.
Added missing check for int_ip.
miniupnpd/pcpserver.c: Added checks for third-party allowed for it to be used.

If allowed, checking it against source address,
with inverse logic from that of non-thirdparty case.
miniupnpd/pcpserver.c: Preliminary work for PCP fw control

Added preliminary is_fw flag, and added af to getifaddr_in6. Made
option parsing follow the RFC and also made it bit more paranoid
(there were some security problems with length checks not being done
at right place all the time; simplified flow, should be easier to
verify now that it does nothing untoward).