Do not allow negative timeouts for kqueue timers, check for the
negative timeout both before and after the conversion to sbintime_t.

For periodic kqueue timer, convert zero timeout into 1ms, to avoid
interrupt storm on fast event timers.

Reported and tested by:	pho
Discussed with:	mav
Reviewed by:	davide
Sponsored by:	The FreeBSD Foundation
Approved by:	re (marius)