This CL starts to introduce IPv6 scoped addressing capability
into the net package.

The Public API changes are:
+pkg net, type IPAddr struct, Zone string
+pkg net, type IPNet struct, Zone string
+pkg net, type TCPAddr struct, Zone string
+pkg net, type UDPAddr struct, Zone string

Update #4234.

R=rsc, bradfitz, iant
CC=golang-dev
https://golang.org/cl/6849045