crypto/tls: add server-side OCSP stapling support.

We already had support on the client side. I also changed the name of
the flag in the ServerHello structure to match the name of the same
flag in the ClientHello (ocspStapling).

R=bradfitzgo
CC=golang-dev
https://golang.org/cl/4408044

src/pkg/crypto/tls/handshake_client.go

@@ -145,7 +145,7 @@ func (c *Conn) clientHandshake() os.Error {
 
 	c.peerCertificates = certs
 
-	if serverHello.certStatus {
+	if serverHello.ocspStapling {
 		msg, err = c.readHandshake()
 		if err != nil {
 			return err