core: make _input_buffer and _output_buffer attributes of http_request class public (to simplify debugging)

core: make http._requests attribute public (to simplify debugging, not because it made it into public interface)

currently it has no effect because there are not 'long' constants but I'd like not to depend on that.

test_threading_2.py: make sure we use gevent in spawned subprocesses as well; this fixes a number of failures

socket: fix typo and possible NameError in connect_ex: it should be EAGAIN, not EGAIN. Patch by Alexey Borzenkov.