This CL makes both InterfaceAddrs and Addrs method on Interface
return IPNet struct for representing interface address and mask
like below:

interface "lo0": flags "up|loopback|multicast", ifindex 1, mtu 16384
        interface address "fe80::1/64"
        interface address "127.0.0.1/8"
        interface address "::1/128"
        joined group address "ff02::fb"
        joined group address "224.0.0.251"
        joined group address "ff02::2:65d0:d71e"
        joined group address "224.0.0.1"
 	joined group address "ff01::1"
        joined group address "ff02::1"
        joined group address "ff02::1:ff00:1"

Fixes #2571.

R=rsc
CC=golang-dev
https://golang.org/cl/5489062