crypto/tls: better error message when connecting to SSLv3 servers.

We support SSLv3 as a server but not as a client (and we don't want to
support it as a client). This change fixes the error message when
connecting to an SSLv3 server since SSLv3 support on the server side
made mutualVersion accept SSLv3.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5545073

handshake_client.go

@@ -59,7 +59,8 @@ func (c *Conn) clientHandshake() error {
 	finishedHash.Write(serverHello.marshal())
 
 	vers, ok := mutualVersion(serverHello.vers)
-	if !ok {
+	if !ok || vers < versionTLS10 {
+		// TLS 1.0 is the minimum version supported as a client.
 		return c.sendAlert(alertProtocolVersion)
 	}
 	c.vers = vers