Get the callback earlier set by calling
gsasl_client_callback_password_set().
RETURN VALUE
Returns the callback earlier set by calling
gsasl_client_callback_password_set().
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.