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/common.go

@@ -178,6 +178,9 @@ func (c *Config) cipherSuites() []uint16 {
 type Certificate struct {
 	Certificate [][]byte
 	PrivateKey  *rsa.PrivateKey
+	// OCSPStaple contains an optional OCSP response which will be served
+	// to clients that request it.
+	OCSPStaple []byte
 }
 
 // A TLS record.