Use recvmsg() and control messages IP_PKTINFO (Linux, OSX, Windows)
or IP_ORIGDSTADDR and IP_RECVIF (RTEMS w/ libbsd) to find the
index of the logical interface through which a UDP packet was received,
and the destination address from the IPv4 header.
Also, clear IP_MULTICAST_ALL on Linux to disable non-compliant legacy brokenness
and receive only those multicasts we are interested in.