ssh: implement partial success in server

[peterverraedt]

Add support for sending back partial success to the client while
handling authentication in the server. This is implemented by a special
error that can be returned by any of the authentication methods, which
contains the authentication methods to offer next. This way a server
can be implemented that e.g. only accepts public keys for the initial
authentication, and only after partial success presents a keyboard
interactive authentication method.

Signed-off-by: Peter Verraedt [email protected]

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[drakkan]

Hi,

I sent a similar patch here. I think we need to wrote a proposal for the API change before a patch can be accepted