BPO-17561: set create_server backlog default to None (GH-12735)

It turns out doing socket.listen(0) does not equal to "choose a
reasonable default". It actually means "set backlog to 0".
As such set backlog=None as the default for socket.create_server.
Fixes the following BB failures:
https://github.com/python/cpython/pull/11784#issuecomment-481036369
Ref. BPO-1756, GH-11784.