Implement NewIPv6UDPConn for Windows

[erikgeiser]

I just opened #431 to make server6 compile on Windows by adding a dummy NewIPv6UDPConn function like in server4. However, it would be awesome if someone could actually implement this function for Windows. I did some research but I can't say that I'm prepared to do so myself.

[pmazzini]

#497 Is related