This CL adds the netaddr interface that will carry a single network
endpoint address or 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 #3610
Update #5267

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13368044