ssh: add support for banners

According to RFC 4252 section 5.4, the banner is sent between the
ssh-connection request and responding to user authentication.

Original support for server sending banner by joshua stein <jcs@jcs.org>

Fixes golang/go#19567

Change-Id: I729b3c8e5fd2c0068609d1590b61e92f40d87ea4
Reviewed-on: https://go-review.googlesource.com/71790
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/test/banner_test.go

@@ -0,0 +1,33 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package test
+
+import (
+	"testing"
+
+)
+
+func TestBannerCallbackAgainstOpenSSH(t *testing.T) {
+	server := newServer(t)
+	defer server.Shutdown()
+
+	clientConf := clientConfig()
+
+	var receivedBanner string
+	clientConf.BannerCallback = func(message string) error {
+		receivedBanner = message
+		return nil
+	}
+
+	conn := server.Dial(clientConf)
+	defer conn.Close()
+
+	expected := "Server Banner"
+	if receivedBanner != expected {
+		t.Fatalf("got %v; want %v", receivedBanner, expected)
+	}
+}