Specify the callback function to use in the server for deciding if
user is authenticated using authentication identity, authorization
identity and password. The function can be later retrieved using
gsasl_server_callback_retrieve_get().
DEPRECATED
This function is part of the old callback interface.
The new interface uses gsasl_callback_set() to set the application
callback, and uses gsasl_callback() or gsasl_property_get() to
invoke the callback for certain properties.