#382 Add load/unload hooks to RexxThreads

[Rony G. Flatscher]

If there was a possibility for an optional load and unload hook for RexxThreads, then some thread-dependent implementations in native code would become much easier, but also much more efficient.

[Comparable to the package loader optional load/unload function, which get a RexxThreadContext.]

[Rony G. Flatscher]

Sorry, pressed the button too quickly:

The "load" hook should be called after the RexxThread got set up, but right before it starts to run, supplying a RexxThreadContext.

The "unload" hook should be called after the RexxThread ran to completion, supplying a RexxThreadContext.