OpenBSD doesn't implement EVFILT_USER filter for kqueue(2), so we couldn't use that for event loop.

instead, use a EVFILT_TIMER filter with EV_ONESHOT (trigger only once) and delay 0sec (which trigger immediatly).
it fits the usage of EVFILT_USER which is only used to "wakeup" the kevent(2) call from userland.