kqueue will remove pipes automatically if their read end is closed.
Unfortunately this means that trying to unregister it (which is
necessary to clean up resources & for consistency with other ioselectors
implementations) will set an ENOENT error, which currently raises an
exception.
(ETA: in other words, it is currently impossible to call unregister on a
pipe fd without potentially getting the selector into an invalid state
on platforms with kqueue.)
Avoid this issue by ignoring ENOENT errors returned from kqueue.
(Tested on FreeBSD. I added a test case to the tioselectors file; the
seemingly unrelated change is to fix a race condition that doesn't
appear on Linux, so that it would run my code too.)
29a7d60acb