bpo-29255: Wait in KqueueSelector when no fds are registered

[russelldavis]

Also partially fixes bpo-25680 (there's still a discrepancy in behavior on Windows that needs to be fixed).

https://bugs.python.org/issue29255

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[matklad]

Also fixes bpo-25680

Not entirely sure, but I think that also needs a separate fix for windows, as all of Linux, Mac and windows behave differently in the case of empty set of selected events. I think this pr only fixes Mac/Linux discrepancy.

[russelldavis]

@matklad thanks for the comment, you're right. I just confirmed that on windows, it raises an exception. I've updated the PR description to reflect that.

[gvanrossum]

I'm not very familiar with the kqueue API, but I looked through the source code for select_kqueue_control_impl and it looks like maxevents is a limit on the number of requested events to be returned from the call, and it looks like it will work correctly -- since there are no fds registered we won't get any events back, but we will get a timeout.