ssh: add (*Client).DialContext method

This change adds DialContext to ssh.Client, which opens a TCP-IP
connection tunneled over the SSH connection. This is useful for
proxying network connections, e.g. setting
(net/http.Transport).DialContext.

Fixes golang/go#20288.

Change-Id: I110494c00962424ea803065535ebe2209364ac27
GitHub-Last-Rev: 3176984a71a9a1422702e3a071340ecfff71ff62
GitHub-Pull-Request: golang/crypto#260
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/504735
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Commit-Queue: Nicola Murino <nicola.murino@gmail.com>

ssh/test/dial_unix_test.go

@@ -9,6 +9,7 @@ package test
 // direct-tcpip and direct-streamlocal functional tests
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net"
@@ -46,7 +47,11 @@ func testDial(t *testing.T, n, listenAddr string, x dialTester) {
 		}
 	}()
 
-	conn, err := sshConn.Dial(n, l.Addr().String())
+	ctx, cancel := context.WithCancel(context.Background())
+	conn, err := sshConn.DialContext(ctx, n, l.Addr().String())
+	// Canceling the context after dial should have no effect
+	// on the opened connection.
+	cancel()
 	if err != nil {
 		t.Fatalf("Dial: %v", err)
 	}