- re_process_buffer no longer writes the modified buffer

  to the client, which was very ugly. It now returns the
  buffer, which it is then written by chat.

- content_length now adjusts the Content-Length: header
  for modified documents rather than crunch()ing it.
  (Length info in csp->content_length, which is 0 for
  unmodified documents)

- For this to work, sed() is called twice when filtering.