Handling ICMP messages in UdpSocket::recv/recv_from

[Mygod]

When running a UDP server, it is helpful to learn if we got ICMP messages (e.g. regarding MTU or unreachability). On Unix, we can do this via recvmsg, e.g. here. It would be great if rust std can incorporate this.

[jonas-schievink]

This should be prototyped in an external crate. Does Windows have similar functionality?