Sadly this exposes the fact that libuv doesn't allow multiple handles
for a given poll fd. Nor does it raise clear errors until you try to
close them in an order different from the inverse of the one they were
created in. Doing so simply aborts the process when libuv realises that
the watcher it has registered is not the watcher you're closing.

This further means that multiple watchers wont' get the right set of
events delivered, I think (though most of the tests were passing until I
discovered this crash).

Sadly, libuv simply cannot be used in a child process that has fork()'d
from a parent that also used libuv. The result is a pretty quick abort()
and crash.

It *may* be possible to implement `subprocess` on top of uv_spawn (I
haven't looked into that), but things like multiprocessing and
futures.ProcessPoolExecutor would be completely broken.

Comments in commit.

fixes test__loop_callback.

The test_threading_2 failure was due to CFFI under Python 3; that combo
hadn't actually been tested before. There's a detailed comment in this
commit that explains the failure and the workaround.

All the tests pass except for
test_threading_2.ThreadJoinOnShutdown.test_3_join_in_forked_from_thread.

This is failing because threading._shutdown is failing. That in turn is
failing because threading._after_fork is getting called.

Now, threading._after_fork is *supposed to always be called*. This is
handled directly by os.fork(). But instrumentation shows that it doesn't
get called when we use the C library (and if we do call it, the test
fails), and apparently it didn't get called before the refactoring. Not
sure what's going on.

[skip ci]

Fixes #222.

Rather than provide a messy monkey-patch on top of `fcntl`, it's simpler
just to mention this attribute, which seems to be stable, and has at
least one reported use of working.

[skip ci]

This time, I saw a bunch of errors like this, which maybe suggest something is
leaking between tests, but I don't know what:

  ===================================================
  ERROR: test_quit (__main__.Test)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "C:\projects\gevent\src\greentest\greentest.py", line 289, in wrap_restore_handle_error
      return method(self, *args, **kwargs)
    File "C:\projects\gevent\src\greentest\greentest.py", line 278, in wrap_error_fatal
      return method(self, *args, **kwargs)
    File "C:\projects\gevent\src\greentest\greentest.py", line 161, in wrap_timeout
      return method(self, *args, **kwargs)
    File "test__backdoor.py", line 75, in test_quit
      conn = self._create_connection()
    File "test__backdoor.py", line 46, in _create_connection
      conn.connect(('127.0.0.1', self._server.server_port))
    File "C:\Python34-x64\lib\site-packages\gevent\_socket3.py", line 291, in connect
      r = getaddrinfo(address[0], address[1], self.family)
    File "C:\Python34-x64\lib\site-packages\gevent\_socketcommon.py", line 272, in getaddrinfo
      return get_hub().resolver.getaddrinfo(host, port, family, socktype, proto, flags)
    File "C:\Python34-x64\lib\site-packages\gevent\resolver_thread.py", line 65, in getaddrinfo
      return self.pool.apply(_socket.getaddrinfo, args, kwargs)
    File "C:\Python34-x64\lib\site-packages\gevent\pool.py", line 326, in apply
      return self.spawn(func, *args, **kwds).get()
    File "C:\Python34-x64\lib\site-packages\gevent\event.py", line 382, in get
      self._wait_core(timeout, ())
    File "C:\Python34-x64\lib\site-packages\gevent\event.py", line 106, in _wait_core
      result = self.hub.switch()
    File "C:\projects\gevent\src\greentest\greentest.py", line 468, in switch
      return _original_Hub.switch(self, *args)
    File "C:\Python34-x64\lib\site-packages\gevent\hub.py", line 630, in switch
      return RawGreenlet.switch(self)
  OSError: [Errno 9] File descriptor was closed in another greenlet

  ----------------------------------------------------------------------
  Ran 6 tests in 10.319s