UDP Server now accepts multicast destination IPs, for example 192.168.1.255.

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.
Allow UDP servers to listen for all addresses or a specific one.