Fix #1044 by always closing opened sockets before raising

Account for some more warnings that Python 3.6 can generate: ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__. See https://travis-ci.org/gevent/gevent/jobs/310040846

Also enable ResoureWarnings by default in the test suite and fix a
bunch that showed up.

Make universal newlines with FileObjectPosix return str on Python 2 i…

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