TODO

* Check the 'flags' field in the 'kevent' struct for EV_ERROR when the
  'kevent' system call returns.

* Is it really worth using a dynamic array to store events returned
  from poll instead of using a fixed size of e.g. 1024?  At least for
  the epoll and kqueue backends that don't require the array size to
  be equal to the number of watched file descriptors.

* Can we avoid writing a byte into the wakeup pipe by first checking
  an IORef?  What are the possible races?

* Return interesting statistics than can be logged by the RTS and used
  in e.g. ThreadScope.  Number of file descriptors returned from a
  poll and the number of timed out polls are two interesting
  statistics.

* How does epoll handle closed file descriptors? They need to be
  removed from the epoll set.

* Change how the default backend is selected as whether you have a
  backend or not might be distrinct from whether that backend is
  functional or not.