net: add addrList

This CL adds a new type addrList that will carry a short list of IP
addresses to dial helper functions in the upcoming CLs.

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/13241046