kevent: Add precision and abstimer flags for EVFILT_TIMER

Add timer precision flags NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS
and NOTE_NSECONDS for EVFILT_TIMER. Also, add an initial implementation

kevent: Add precision and abstimer flags for EVFILT_TIMER

Add timer precision flags NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS
and NOTE_NSECONDS for EVFILT_TIMER. Also, add an initial implementation
of NOTE_ABSTIME timers.

Similar kevent(2) flags exist on FreeBSD, NetBSD and XNU.

Initial diff by and OK aisha@
OK mpi@

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Exercise resetting of expired and unexpired timers.

kevent: correctly check that timeout's nanoseconds are on [0, 1000000000)

Validate the input with timespecfix before truncating to a timeval.
timespecfix does not round, so we need to to it by hand

kevent: correctly check that timeout's nanoseconds are on [0, 1000000000)

Validate the input with timespecfix before truncating to a timeval.
timespecfix does not round, so we need to to it by hand after validation.

FreeBSD and NetBSD check the input with this range, we ought to as well.

Also add a regression test for this case.

ok tb@

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To make debugging the kqueue test easier, always print the assertion
failure before returning.

simplistic regress test for KEVENT_TIMER kqueue(2) calls

ok and prodding tedu@