os: fix WNOHANG Waitmsg

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4962042

src/pkg/os/exec_unix.go

@@ -38,7 +38,8 @@ func (p *Process) Wait(options int) (w *Waitmsg, err Error) {
 	if e != 0 {
 		return nil, NewSyscallError("wait", e)
 	}
-	if options&WSTOPPED == 0 {
+	// With WNOHANG pid is 0 if child has not exited.
+	if pid1 != 0 && options&WSTOPPED == 0 {
 		p.done = true
 	}
 	w = new(Waitmsg)