Auto merge of #3829 - tiif:edgecase, r=oli-obk

epoll: handle edge case for epoll_ctl

There is a test case that revealed that our implementation differs from the real system:
 - Set up an epoll watching the FD
- Call epoll_wait
- Set up another epoll watching the same FD
- Call epoll_wait on the first epoll. Nothing should be reported!

This happened because, in ``epoll_ctl``, we used ``check_and_update_readiness``, which is a function that would return notification for all epoll file description that registered a particular file description. But we shouldn't do that because no notification should be returned if there is no I/O activity between two ``epoll_wait`` (every first ``epoll_wait`` that happens after ``epoll_ctl`` is an exception, we should return notification that reflects the readiness of file description).

r? `@oli-obk`

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