net: add netaddr interface

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 issue 3610
Update issue 5267

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13368044
misc/dist: remove Xcode files from the darwin binary distribution

Fixes issue 6132.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/12858049

Committer: Andrew Gerrand <adg@golang.org>