Don't "forget" listener options

If a listener is started or stopped via `ejabberd_listener:add_listener/3` or `ejabberd_listener:delete_listener/3`, the configuration for all listener modules is updated using the `Module:transform_listen_option/2` function for each listener module that exports such a function.  However, for listener modules that don't provide that function (such as `ejabberd_stun`), all but one option was dropped.  This is now fixed.

The issue could be triggered e.g. by enabling `mod_proxy65` in the `modules` section.