net: make channel-based semaphore depend on receive, not send

R=r, dvyukov
CC=golang-dev
https://codereview.appspot.com/13348045

Committer: Rob Pike <r@golang.org>
cmd/go: make it work for code.google.com cloned repositories

Thanks to beatgammit for the fix.

Fixes issue 5408.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13261048