Multicast support for its new QUdpSocket class in Qt4

Please consider adding Multicast support for the QUdpSocket class. Most of the desired functionality comes from additional ::setsockopt calls for items such as:

setting multicast loopback (IP_MULTICAST_LOOP)
setting multicast TTL (IP_MULTICAST_TTL)
setting the multicast interface (IP_MULTICAST_IF)
joining a multicast group (IP_ADD_MEMBERSHIP)
leaving a multicast group (IP_DROP_MEMBERSHIP)

Because of the O/S differences in bind for multicast it might be necessary to also reimplement bind (trying "bind( address, port)" first and, if this failed, trying "bind(QHostAdddres( (Q_UINT32)0), port)".). Linux, BSD, and others require the first to limit Multicast traffic to the provided address. Windows will fail on this call, but the second will succeed. For the latter, joining the group limits the packets.