The configuration string is appended to the XAuth backend name, separated by
a colon. The configuration string is passed untouched to the backend, where
it can change the behavior of the XAuth module.
This XAuth backend does not do any authentication of client credentials
but simply sends a successful XAuth status to the client, thereby
concluding the XAuth exchange. This can be useful to fallback to basic
RSA authentication with clients that can not be configured without XAuth
authentication.