Register Event Callback Node is not supported on non-Windows platforms

[logmanoriginal]

After reviewing this topic several times over the past weeks, the issue is finally clear to me. Turns out, Register Event Callback Node is actually supposed to be used only for .NET and ActiveX events, even though it clearly also works for user events. Whether this is intended behavior I can't say but it is certainly undocumented...

Use this function to register .NET and ActiveX events only.
-- https://www.ni.com/docs/en-US/bundle/labview-api-ref/page/functions/register-event-callback.html

For the purpose of user events, Register For Events should be used.

This will require some refactoring to fix all calling sites. In the meantime, there will be a hotfix to disable callback refnums on non-Windows platforms.

Originally posted by @logmanoriginal in #23

[logmanoriginal]

When PostLVUserEvent is called on a user event that is registered with Register For Events the execution of PostLVUserEvent only blocks long enough to safely copy the data. When a user event is registered with Register Event Callback, however, the callback VI is executed in-place and PostLVUserEvent blocks until the callback VI is finished. The use of Register For Events is intended; the behavior of Register Event Callback is coincidental.

See also:
https://lavag.org/topic/19251-labview-vi-and-c-callback/#findComment-116130

There is, unfortunately, no other way besides PostLVUserEvent to implement callbacks in LabVIEW. For use with Register For Events, additional synchronization is required. Of course, there is no built-in support.