internet: (fixes #1040) Multicast receive overhaul

I recently found that multicast packet reception is severely questionable.

This is a tentative to make it more logical (but still somewhat close to Linux).

To receive a multicast packet you'll have to:

• Open a socket
• Bind to an address (ANY or a unicast address)
• Call UdpSocket::MulticastJoinGroup(targetAddr, interfaceIndex); (or use 0 for all the interfaces)

or

• Open a socket
• Bind to a multicast address

To be done:

• Remove the multicast address from IPv4 when the endpoint (or the socket) is removed
• Mirror the changes to IPv6
• Add multicast support to Ipv[4,6]RawSocket
• Write a test
• Write an example
• Documentation

To be tested:

• What happens if you try to send a packet over a socket bound to a multicast address (probably nothing good).

Tried - for the records the simulation hangs. A socket bound to a multicast address can only receive data, obviously. I might (or might not) force the receive-only behavior. I think I'll not force it, so that we'll spot people that don't think.