selectors: Plumb in kqueue user events instead of pipes for custom events

[elcritch]

This enables using kQueue EVFILT_USER for much faster user triggered events on MacOSX and likely *BSDs. Currently gated behind a new kqueue flag -d:kqueueUserEvent.

It's a little tricky to make it so the trigger function works on a kqueue custom event. I handled this by only enabling triggering once a custom event has been registered with a selector and is given that selector's kqueue FD. This may cause a bit of a race condition when closing that selector, though similar issues occur with pipe based events as well.

There's a bit of a question I had about whether a SelectEvent should support being added to multiple selectors. The current implementation only allows a user event to be registered with a single selector. On Linux SelectEvent's use eventfd and should work fine with multiple selectors & threads. Given that, it might be best to enable the kqueue-based user events to be registered with multiple selectors. That'd require using a sharedArray of kqueue FD's.

[elcritch]

Note, I'd like to add a test before this is merged. However I wanted to open the PR so I could get feedback on whether multiple selector's should be supported.

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This pull request has been marked as stale and closed due to inactivity after 395 days.