Comments inline.

[skip ci]

We now depend on CFFI/Python new enough that it should be built during
setup/installation.

Trying to build at runtime easily results in "gevent" directories
scattered all over the system.

This makes the two branches equivalent in terms of scheduling. Fixes
notably test_ftplib.py which relies on running a "threaded" server.

- Compliance: If :func:`gevent.select.select` is given a negative *timeout*
  argument, raise an exception like the standard library does.
- Compliance: If :func:`gevent.select.select` is given closed or invalid
  file descriptors in any of its lists, raise the appropriate
  ``EBADF`` exception like the standard library does. Previously,
  libev would tend to return the descriptor as ready. In the worst
  case, this adds an extra system call, but may also reduce latency if
  descriptors are ready at the time of entry.
- Compliance: :meth:`gevent.select.poll.unregister` raises an exception if *fd* is not
  registered, like the standard library.
- Compliance: :meth:`gevent.select.poll.poll` returns an event with
  ``POLLNVAL`` for registered fds that are invalid. Previously it
  would tend to report both read and write events.

This serves to be better documenting about what they're doing and why,
instead of having a dozen slightly different loops that each have to be
examined.

Fix #775

Fix test_socket under 3.4 with the ares resolver by handling in Python code two cases ares itself doesn't handle.

Return whether the pool became empty in join.

Useful with a timeout argument.

Fixes #503.

[skip ci]

It would become False anyway as soon as the method returned. We won't
schedule another one while it's running (True), so there's no race
condition.

Switch to using prospector for lint checks

Don't immediately run links added to an Event/AsyncResult while it is running callbacks. Fixes #771.