ssh: allow to bind to a hostname in remote forwarding

To avoid breaking backwards compatibility, we fix Listen, which
receives the address as a string, while ListenTCP can still only
be used with IP addresses.

Fixes golang/go#33227
Fixes golang/go#37239

Change-Id: I4d45b40fdcb0d6012ed8da59a02149fa37e7db50
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/599995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Bishakh Ghosh <ghoshbishakh@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

ssh/streamlocal.go

@@ -44,7 +44,7 @@ func (c *Client) ListenUnix(socketPath string) (net.Listener, error) {
 	if !ok {
 		return nil, errors.New("ssh: streamlocal-forward@openssh.com request denied by peer")
 	}
-	ch := c.forwards.add(&net.UnixAddr{Name: socketPath, Net: "unix"})
+	ch := c.forwards.add("unix", socketPath)
 
 	return &unixListener{socketPath, c, ch}, nil
 }
@@ -96,7 +96,7 @@ func (l *unixListener) Accept() (net.Conn, error) {
 // Close closes the listener.
 func (l *unixListener) Close() error {
 	// this also closes the listener.
-	l.conn.forwards.remove(&net.UnixAddr{Name: l.socketPath, Net: "unix"})
+	l.conn.forwards.remove("unix", l.socketPath)
 	m := streamLocalChannelForwardMsg{
 		l.socketPath,
 	}