[BUG] O(1) pollers should check their FD before closing it

[BUG] O(1) pollers should check their FD before closing it

epoll, sepoll and kqueue pollers should check that their fd is not
closed before attempting to close it, otherwise we can end up with
multiple closes of fd #0 upon exit, which is harmless but dirty.

(cherry-picked from commit d79e79b436144654d10124de7d5fd4c896ac0487)
(cherry picked from commit 0d27ec2050a3208150af5dcaeda9dab7a53fdaf7)