Support for UDPv4/v6 multicast streaming

Support for UDPv4/v6 multicast streaming

This provides self-contained support for UDPv4/v6 multicast:
 * receiver joins multicast v4/v6 address;
 * sender, on encountering a multicast address, sets TTL/IPv6
   number of multicast hops, and (as a precaution) enables looping
   back of multicast packets.

The use of the sender is still restricted to UDPv4, since it is also
necessary to perform changes to the (IPv4-based) udp_target struct,
which should be done in a separate patch.