lots of simplifications

- Split pipes are now implemented with per-output subpipes, on which standard
  linear commands (set_output, set_ubuf_mgr, etc.) apply. Therefore there
  is no need for the "linear" prefix any longer.
- upipe_input no longer uses upipe_control because the semantics are
  different (the argument belongs to the callee). Also a upump argument
  has been added for future use.
- A connection between pipes may only carry one flow. Therefore flow names
  and flow deletion packets are not needed any longer. Use upipe_release to
  delete a flow.
- New DEAD event thrown when a pipe is released.