os/exec: add Cmd.Waitmsg, fix a misleading comment
os/exec: add Cmd.Waitmsg, fix a misleading comment

Fixes issue 2948

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5655048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go