go.crypto/ssh: ensure {Server,Client}Conn do not expose io.ReadWriter

Transport should not be a ReadWriter. It can only write packets, i.e. no partial reads or writes. Furthermore, you can currently do ClientConn.Write() while the connection is live, which sends raw bytes over the connection. Doing so will confuse the transports because the data is not encrypted.

As a consequence, ClientConn and ServerConn stop being a net.Conn

Finally, ensure that {Server,Client}Conn implement LocalAddr and RemoteAddr methods that previously were exposed by an embedded net.Conn field.

R=hanwen
CC=golang-dev
https://golang.org/cl/16610043

ssh/tcpip.go

@@ -296,7 +296,7 @@ type channelOpenDirectMsg struct {
 // strings and are expected to be resolvable at the remote end.
 func (c *ClientConn) dial(laddr string, lport int, raddr string, rport int) (*tcpChan, error) {
 	ch := c.newChan(c.transport)
-	if err := c.writePacket(marshal(msgChannelOpen, channelOpenDirectMsg{
+	if err := c.transport.writePacket(marshal(msgChannelOpen, channelOpenDirectMsg{
 		ChanType:      "direct-tcpip",
 		PeersId:       ch.localId,
 		PeersWindow:   1 << 14,