setup.py: add "build_ext --cython=" option, which allows users to specify the path to the cython executable.

this also allows users to skip automatic cython compilation completely
by specifying an empty cython executable, e.g. by providing the
following setup.cfg:

,----[ setup.cfg ]
| [build_ext]
| cython=
`----

I use this to manage a copy of gevent inside another git repository
while at the same time making sure that core.c doesn't get
regenerated.

pywsgi.py: close the client socket after having handled all requests. do not rely on garbage collection.

test__pywsgi.py: test that sockets are closed even if the wsgi app leaks the environ, the wsgi.input object or keeps a reference to a frame.

socket: fix getaddrinfo to handle international domain names (instead of calling stdlib's getaddrinfo)

core: remove EVENT_INTERNAL_AVAILABLE-related stuff. libevent-internal.h is not up to date with the latest libevent anyway.

the releases will continue to include it but it no longer useful to keep it in the repo as it's much easier to build now, setup.py will do that automatically.