Specify the callback function to use in the client to set the
anonymous token, which usually is the users email address. The
function can be later retrieved using
gsasl_client_callback_anonymous_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.