Run distribute-tasks interactively and have the operator confirm

any changes.  Better handling of changed origins.

* distribute-tasks
  (JobInfo.__init__): Initialize __active.
  (JobInfo.set_active): New method.
  (JobInfo.active): New method.
  (JobInfo.destination): New method.
  (tasks_per_destination): New global variable.
  (interactive): New global variable.
  (read_tasks): Update tasks_per_destination.
  (read_new_tasks): Use JobInfo.source() to simplify code.  Update
    tasks_per_destination.  Set lysrdiffpart of info if the source is
    already backed up, to avoid moving a backed up directory to a
    different partition.  Report a fatal error if the destination
    changes.
  (write_task_lists): Only skip jobs if running interactively.  Only
    reorder tasks if running non-interactively.  Have the user confirm
    changes if running interactively.
  (main): New argument: -i (--interactive).  Only read new tasks if
    running interactively.