ssh: add openssh controlmaster socket support

Adds support for establishing SSH sessions over an existing "ControlMaster" [unix domain] socket in proxy mode.

Details of the protocol can be found here: https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.mux
More details about ControlMaster sockets can be found here: https://linux.die.net/man/5/ssh_config

Fixes golang/go#32958

Co-authored-by: Cyrus Katrak <ckatrak@slack-corp.com>
Change-Id: Ia3ae8893262f5060ed3fadfcbe97619c9659145b
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/733040
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

ssh/connection.go

@@ -91,9 +91,17 @@ func DiscardRequests(in <-chan *Request) {
 	}
 }
 
+// A connTransport represents the transport for a connection.
+type connTransport interface {
+	packetConn
+	getAlgorithms() NegotiatedAlgorithms
+	getSessionID() []byte
+	waitSession() error
+}
+
 // A connection represents an incoming connection.
 type connection struct {
-	transport *handshakeTransport
+	transport connTransport
 	sshConn
 
 	// The connection protocol.