ObjFW  Check-in [220513a3f5]

This was necessary because select(), poll() and kevent() on FreeBSD and
OS X would sometimes return 0 events, even if there are some, and
sometimes return the correct number of events that were pending, meaning
the number of events returned is unreliable. To make things worse,
whether it returns 0 or the number of events that were pending is
completely non-deterministic on both FreeBSD and OS X (running the same
tests multiple times in a row would make it sometimes work and sometimes
fail).

In order to prevent code from depending on the return value of
-[observeForTimeInterval:] (which would depend on select(), poll() and
kevent() returning the correct number), OFKernelObserver no longer
returns whether there were pending events. It is expected that
-[observe] or -[observeForTimeInterval:] is just called in a loop as
long as events should be handled.

The tests have been changed as well to reflect this. What they do now is
set a deadline and call -[observeForTimeInterval:] with a small timeout
in a loop until the deadline is reached or all events have been handled.

Note: DragonFlyBSD has not been tested, but will most likely behave like
FreeBSD and OS X.