* Change handling of ^G interrupts and of ABORT-CURRENT-COMMAND. Both

* Change handling of ^G interrupts and of ABORT-CURRENT-COMMAND.  Both
  now use the condition system; ^G conditions are a specialization of
  ABORT-CURRENT-COMMAND conditions.  This change makes it easy to bind
  some action to occur when a command is aborted for whatever reason.
  Consequently, the procedure INTERCEPT-^G-INTERRUPTS has been
  deleted.

* The inferior thread output mechanism has been modified to allow a
  thread to request that the editor exit the keyboard reader and
  return to the command reader.  The request is phrased by the thread
  output procedure returning 'FORCE-RETURN.  This new mechanism is
  used by the inferior REPL code to force the command reader to
  immediately execute a command override for an unsolicited prompt.

* Aborting an unsolicited prompt causes the associated inferior thread
  to execute ABORT->NEAREST.

* Inferior REPL buffers now initialize their working directory to the
  default directory of the selected buffer at the time the REPL buffer
  is created.

* Inferior REPL buffers now have their own bindings of %EXIT and QUIT
  that affect only the inferior thread.  In particular, %EXIT kills
  the inferior thread but leaves Scheme running; QUIT does nothing.