crypto/tls: add client-side SNI support and PeerCertificates.

SNI (Server Name Indication) is a way for a TLS client to
indicate to the server which name it knows the server by. This
allows the server to have several names and return the correct
certificate for each (virtual hosting).

PeerCertificates returns the list of certificates presented by
server.

R=r
CC=golang-dev
https://golang.org/cl/1741053

src/pkg/crypto/tls/handshake_client.go

@@ -28,6 +28,7 @@ func (c *Conn) clientHandshake() os.Error {
 		compressionMethods: []uint8{compressionNone},
 		random:             make([]byte, 32),
 		ocspStapling:       true,
+		serverName:         c.config.ServerName,
 	}
 
 	t := uint32(c.config.Time())
@@ -107,6 +108,8 @@ func (c *Conn) clientHandshake() os.Error {
 		return c.sendAlert(alertUnsupportedCertificate)
 	}
 
+	c.peerCertificates = certs
+
 	if serverHello.certStatus {
 		msg, err = c.readHandshake()
 		if err != nil {