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

Hopefully this will help diagnose the timeout error seen (only) on
Appveyor: The test that runs after test_multi hangs; all the other tests
work. (This was verified by changing the name of test_multi and thus its
sort order; the test after it is the one that fails.)