Use authMethodsLeft in the sample servers

[artdent]

RFC 4252, which covers ssh authentication, allows clients to respond to request the auth method 'none', to which the server should respond with a list of allowed methods. In particular, WinSCP performs this check, and if the server responds with an empty list, it terminates the session. (Most other clients just try various methods without checking.)

So, it'll be good to handle this case in the sample server implementations as a hint to implementors about this part of the protocol.