Upgrade c-ares to 1.13.0. Fixes #990

Enable warnings when running our tests. Rename (the undocumented) threadpool.ThreadResult.async to async_watcher because it's a keyword in 3.7, as exposed by warnings.

    * revert Group.add to original state (no new params)
    * add new exception "Full", and return it when blocking==False and
    pool is full
    * no longer return True/False from Pool.add

gevent.queue.__all__ to include Full and Empty

Fix a typo in _socket3.py [skip ci]

[skip appveyor]

More safely terminate process on Windows.

We need to set the result when we detect a process is dead in the act of killing it, so that in the future we don't try to wait on it again.