ssh: also start forward listeners on ListenUnix

Forgotten file from CL 112635.

The integration tests didn't catch this because the integration tests
never run (sshd isn't in $PATH usually), but even when $PATH is
modified, it seems to have been failing for ages on both Debian jessie
and stretch. I'll fix the tests and make them run in TryBots in later
changes.

Change-Id: I85fd3b6109a73990bc353a61da5f527b9a698501
Reviewed-on: https://go-review.googlesource.com/113056
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/streamlocal.go

@@ -32,6 +32,7 @@ type streamLocalChannelForwardMsg struct {
 
 // ListenUnix is similar to ListenTCP but uses a Unix domain socket.
 func (c *Client) ListenUnix(socketPath string) (net.Listener, error) {
+	c.handleForwardsOnce.Do(c.handleForwards)
 	m := streamLocalChannelForwardMsg{
 		socketPath,
 	}