feat(webrtc): Tell each page the public address it reached the server at
A server published at more than one public address, such as two NAT
entries forwarding the same ports to it, loads its page through any of
them, but WebRTC media flows only to the addresses its host candidates
advertise. webrtc_public_ip is read as a list, yet host candidates
advertised only the first address of each family, so a page that came in
through another entry was told an address it may not be able to reach.
The signaling route now keeps the request's Host header with the peer
(PeerInfo::host). When it names an address webrtc_public_ip lists, that
peer's host candidates advertise it in place of the first of its family
(IceConfig::prefer_public_ip). A host name, an unlisted address or no
header leaves the first address in force, as before. Only the address
changes, not the port, so every entry must forward the same ICE ports.