net: make resolveInternetAddr return a list of addresses

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 issue 3610
Update issue 5267

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13374043