UPnP: Bind to IPv6 addresses for search responses

Older miniupnpd 2.0 will send a SSDP search response with an IPv6 location to a IPv4 address,
but newer ones 2.2 won't. So we need to also bind to an IPv6 address for the SSDP search
to receive the router's IPv6 location. Then we can bind to our public IPv6 address
for a port forward and it will work when miniupnpd is configured for "secure".

Also, don't bind a POST request to a mismatched v4/v6 address.