go.crypto/ssh: prevent channel writes after Close

Fixes golang/go#3810.

This change introduces an atomic boolean to guard the close
of the clientChan. Previously the client code was quite
lax with the ordering of the close messages and could allow
window adjustment or EOF messages to leak after Close had
been signaled.

Consolidating the changes to the serverChan will be handled
in a following CL.

R=agl, kardianos, gustav.paul
CC=golang-dev
https://golang.org/cl/6405064

ssh/session.go

@@ -370,7 +370,7 @@ func (s *Session) stdin() {
 	}
 	s.copyFuncs = append(s.copyFuncs, func() error {
 		_, err := io.Copy(s.clientChan.stdin, s.Stdin)
-		if err1 := s.clientChan.stdin.Close(); err == nil {
+		if err1 := s.clientChan.stdin.Close(); err == nil && err1 != io.EOF {
 			err = err1
 		}
 		return err