net: fix dial race on plan9 and windows

Fixes issue 5349.

R=golang-dev, lucio.dere, dsymonds, bradfitz, iant, adg, dave, r
CC=golang-dev
https://codereview.appspot.com/9159043
cmd/cgo: pass -Wsystem-headers when looking for errors

This works around a bug in GCC 4.8.0.

Fixes issue 5118.

R=golang-dev, r, minux.ma
CC=golang-dev
https://codereview.appspot.com/9120045
doc: pull front page featured articles using new blog JSON feed

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/9288045
spec: fix incorrect example

Fixes issue 5430.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9305043