miniupnpd/pcpserver.c: take care of "nonce" value
miniupnpd/pcpserver.c: use const where useful
miniupnpd/pcpserver.c: prepare code to be able to manage more than just TCP and UDP
miniupnpd/pcpserver.c: add comments
miniupnpd/minissdp.c: reduce syslog() verbosity

LOG_INFO => LOG_DEBUG
miniupnpd: start work to enable IPv6 PCP operations