Support Multicast

[daurnimator]

Add support for IP multicast.

• SO_BROADCAST See Add SO_BROADCAST option #122
• Membership: I'm thinking a key to bind/connect/accept: multicast = {address1, address12, ...}
  This would be translated to the necessary IPV6_ADD_MEMBERSHIP/IP_ADD_MEMBERSHIP calls.
  I don't think DNS lookups should be involved? but I'm happy to be corrected.
• IP{,V6}_MULTICAST_LOOP
• IP_MULTICAST_TTL/IPV6_MULTICAST_HOPS