Add and export utility functions for IPv4 networks: MAKE-SUBNET-MASK,...

Add and export utility functions for IPv4 networks: MAKE-SUBNET-MASK, ADDRESS-NETWORK-PORTION, ADDRESS-HOST-PORTION, ADDRESS-IN-NETWORK-P, ADDRESSES-IN-SAME-NETWORK-P.

Signed-off-by: Stelian Ionescu <sionescu@common-lisp.net>