"Unbuffered" mode of class _fileobject wasn't actually unbuffered,

"Unbuffered" mode of class _fileobject wasn't actually unbuffered,
and this broke a Zope "pipelining" test which read multiple responses
from the same connection (this attaches a new file object to the
socket for each response).  Added a test for this too.

(I want to do some code cleanup too, but I thought I'd first fix
the problem with as little code as possible, and add a unit test
for this case.  So that's what this checkin is about.)