Note that this CL will break your existing code which uses
ParseCIDR.

This CL changes ParseCIDR("172.16.253.121/28") to return
the IP address "172.16.253.121", the network implied by the
network number "172.16.253.112" and mask "255.255.255.240".

R=rsc, borman
CC=golang-dev
https://golang.org/cl/4749043