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.
Merge pull request #232 from weiss/dont-drop-listen-options

Don't "forget" listener options