kqueue: fix recent assertion

NOTE_ABSTIME may also have a zero timeout, which indicates that we
should still fire immediately as an absolute time in the past. A test
has been added for this one as well.

Fixes: 9c999a259f00 ("kqueue: don't arbitrarily restrict long-past...")
Point hat: kevans
Reported by: syzbot+1c8d1154f560b3930042@syzkaller.appspotmail.com