UDP MULTICAST RECEPTION

```
Description: The logic in udp_input() expects either a single receive socket or
none at all. However, multiple sockets should be capable of
receiving a UDP datagram (multicast reception). This could be
handled easily by something like:

for (conn = NULL; conn = udp_active (pbuf, conn); )

If the callback logic that receives a packet responds with an
outgoing packet, then it will over-write the received buffer,
however. recvfrom() will not do that, however. We would have
to make that the rule: Recipients of a UDP packet must treat
the packet as read-only.
Status: Open
Priority: Low, unless your logic depends on that behavior.
```