Specify the callback function to use in the client to know which
realm it belongs to. The realm is used by the server to determine
which username and password to use. The function can be later
retrieved using gsasl_client_callback_realm_get().
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.