ssh_options_get can now return ProxyCommand

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
server: fix pubkey reply for key probes

Per RFC 4252, it is required to send back only one of either
SSH_MSG_USERAUTH_PK_OK or SSH_MSG_USERAUTH_FAILURE for public
key probes.

Update the handling of 'auth_pubkey_function' to send back PK_OK
instead of SSH_MSG_USERAUTH_SUCCESS for the case that the state
of the message at hand is SSH_PUBLICKEY_STATE_NONE.

With this change, it is now possible to process an initial key probe
and then subsequent signature validation using the server callbacks.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>