Allow to join multiple multicast groups on UDP

[Mallets]

In case of using UDP multicast, an endpoint configuration option is added to support the reading from multiple multicast groups. E.g.: udp/224.0.0.1#iface=en0;join=224.0.0.2|224.0.0.3 will operate as follows:

• write will be done on iface=en0 and on the 224.0.0.1 multicast group
• read will be done from the following multicast groups: 224.0.0.1, 224.0.0.2, 224.0.0.3