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/test/forward_unix_test.go

@@ -51,6 +51,8 @@ func testPortForward(t *testing.T, n, listenAddr string) {
 		}
 	}()
 
+	// The forwarded address match the listen address because we run the tests
+	// on the same host.
 	forwardedAddr := sshListener.Addr().String()
 	netConn, err := net.Dial(n, forwardedAddr)
 	if err != nil {
@@ -111,6 +113,8 @@ func testPortForward(t *testing.T, n, listenAddr string) {
 }
 
 func TestPortForwardTCP(t *testing.T) {
+	testPortForward(t, "tcp", ":0")
+	testPortForward(t, "tcp", "[::]:0")
 	testPortForward(t, "tcp", "localhost:0")
 }