Whilst preparing & testing PR #3954, I noticed that macOS kevent and (recent) unix-like ppoll operating
system calls would leave their respective os data structures open in case of an Exception. Oops!

The data structure does get closed on the success path.

Issue #3944 shows that these os calls can & do return errno values which then cause Exceptions to be thrown.

The fix is to wrap each call in a try/finally block.

I did not do this in PR #3954 because I wanted reviewers to be able to focus on the essential math change.

Instead, I am creating this Issue as a "forget-me-not" and hope to make the change after #3954 settles.