notes:
Darwin is very particular about joining a multicast group if the
listneing socket is not created in "udp4" mode, the other supported
OS's are more flexible.

A simple example sets up a socket to listen on the mdns/bonjour
group 224.0.0.251:5353

// ensure the sock is udp4, and the IP is a 4 byte IPv4
socket, err := net.ListenUDP("udp4", &net.UDPAddr {
        IP: net.IPv4zero,
        // currently darwin will not allow you to bind to
        // a port if it is already bound to another process
        Port: 5353,
})
if err != nil {
        log.Exitf("listen %s", err)
}
defer socket.Close()
err = socket.JoinGroup(net.IPv4(224, 0, 0, 251))
if err != nil {
        log.Exitf("join group %s", err)
}

R=adg, rsc
CC=golang-dev
https://golang.org/cl/4066044