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/test_unix_test.go

@@ -27,6 +27,7 @@ import (
 
 const sshd_config = `
 Protocol 2
+Banner {{.Dir}}/banner
 HostKey {{.Dir}}/id_rsa
 HostKey {{.Dir}}/id_dsa
 HostKey {{.Dir}}/id_ecdsa
@@ -256,6 +257,8 @@ func newServer(t *testing.T) *server {
 	}
 	f.Close()
 
+	writeFile(filepath.Join(dir, "banner"), []byte("Server Banner"))
+
 	for k, v := range testdata.PEMBytes {
 		filename := "id_" + k
 		writeFile(filepath.Join(dir, filename), v)