Added UDP listening server multicast destination ip support.

Added support for multicast destination IPs, in tcpip.cpp

Split eth_type_is_ip_and_my_ip(word len) into 2 functions:
eth_type_is_ip(word len)
is_my_ip(word len)

Added is_multicast_ip(word len), that checks the destination Ip to see if it is a valid multicast by comparing it with the subnet mask.

Returns 0 if false, non zero otherwise.