Register both IPv4 and IPv6 on the master server

some good news regarding IPv6 rollout over the master servers is that IPv6 seems to work fine server-side right now. however, while experimenting, i've found that dual stack servers are currently unable to advertise on both IPv4 and IPv6, and is instead only advertising on the IPv6 address unless the game is started with -noipv6, which limits it to IPv4 only.

turns out that the simple cause is that the client only sends a request once, prioritizing IPv6 over IPv4 whenever possible. this doesn't really work out since it need to send a request on both IPv4 and IPv6 for both addresses to be visible on the master server. changing this to two requests, one per IP address, fixes the problem.

NOTE: i've only tested and verified it works on a dual stack system, since that's all i have. ipv4-only is still untested, and i would be grateful if someone with such a setup can test this to ensure no regressions are introduced by this PR.