SSDP responds with IPv6 addresses on IPv4 requests

[stuarta]

SSDP has different multicast listen addresses for IPv4 and IPv6

• 239.255.255.250:1900 (IPv4)
• [FF02::C]:1900 - (IPv6 - link local)
• [FF05::C]:1900 - (IPv6 - site local)

Currently we receive an SSDP Discovery request, we respond with the IPv6 addresses on the IPv4 multicast. We should only send IPv4 endpoints in response to requests via IPv4, and IPv6 endpoints in response to requests via IPv6

• Fix responses received on IPv4 multicast
• Add support for IPv6 multicast

[stuarta]

Analysis shows that in fact our response to an M-SEARCH request is wrong. The SPEC says

Multi-homed devices shall send the search response using the same UPnP-enabled interface on which the search request was received. The URL specified in the LOCATION field value shall specify an address that is reachable on that interface.

Our code responds with 1 reply for each and every interface on the system, which is incorrect. This would be correct in the NOTIFY case, where you should SEND a notification out each interface with the LOCATION matching that network