This CL makes resolveInternetAddr return a list of addresses that
contain a pair of different address family IP addresses if possible,
but doesn't contain any API behavioral changes yet. A simple IP
address selection mechanism for Resolve{TCP,UDP,IP}Addr and Dial API
still prefers IPv4.

This is in preparation for TCP connection setup with fast failover on
dual IP stack node as described in RFC 6555.

Update #3610
Update #5267

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13374043