#356 Add shutdown hook capability

[Rony G. Flatscher]

It would be very helpful, if ooRexx had the capability of "shutdown hooks" like it is available for Java.

Some logic like: allow registering (unregistering) shutdown hooks (e.g. like Java), and right before the runtime starts to shutdown (maybe when all Rexx interpreter instances got terminated) - and well before the unloaders get invoked - all added shutdown hooks would get run?