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

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

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go