Fix a warn-and-fail bug in kqueue by providing kevent() room to report errors

Fix a warn-and-fail bug in kqueue by providing kevent() room to report errors

Apparently, kevent fails gracefully if there is not enough space in its
output events array to report every _event_... but it just dies and returns
-1 if there is not enough space to report every _error_.

There are a couple of possible fixes here.  One would to handle -1
returns from kevent better by re-growing the array and retrying... but
that seems a little error prone.  Instead, I'm just going to say that
the events array must be large enough to handle all the errors.

This patch also adds a unit test designed to make sure that our
many-events-out code works even if not all the events are added at
once.