Specify the callback function to use in the client to set the name
of the service. The service buffer should be a registered GSSAPI
host-based service name, hostname the name of the server.
Servicename is used by DIGEST-MD5 and should be the name of generic
server in case of a replicated service. The function can be later
retrieved using gsasl_client_callback_service_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.