Stdio.Buffer.set_buffer_mode revisited

o I would never ever under any circumastances inherit something more
  than once. Instead, have an input and an output buffer object.

  It's a miracle it even works. Note that range_error will be called
  for both buffer objects, how do you know which one it is for?

  See also: All your wrappers for the various read_* functions.

o Why have transmit()? It is handled automatically when data is added
  to the output buffer.

o There is no need to have a write_cb, really, it should be
  pointless. Writing is handled by the file object in buffered mode.


o There seems to be more locking than is strictly needed.
  Is the cond/locks really needed?

  The read/write callbacks will always be called from the backend
  thread (range_error for output buffer and read_cb) so why would they
  need a condition variable to signal to each other?