Only show the SSH banner once, even if the client attempts
authentication with the "none" type and resets the authentication
attempts.
Change-Id: I1a7aacb50abf00233ed4d06c60808aaf51a9d7be
Reviewed-on: https://go-review.googlesource.com/78544
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>