Make sure motion control returns to main thread before stopping (UniversalRobots#490)

When the watchdog times out, we halt in the external_control script and
call stopj() just before that. However, ongoing motion will never happen
in the main thread, but we always use a motion thread. Thus, calling
stopj() here will result in an error that currently another thread is
controlling the robot's motion.

Since we still want to stop the robot using the predefined deceleration,
we have to stop all motion beforehand. Effectively returning motion
control to the main thread before calling stopj().

Alternatively, we could remove the `stopj()` command (it is effectively
not doing anything right now, anyway), but that would result in a very
sudden stop of the robot.

Author: urfeex

resources/external_control.urscript

@@ -1083,6 +1083,13 @@ while control_mode > MODE_STOPPED:
     end
   else:
     textmsg("Socket timed out waiting for command on reverse_socket. The script will exit now.")
+    if control_mode == MODE_FREEDRIVE:
+      end_freedrive_mode()
+    end
+    control_mode = MODE_STOPPED
+    kill thread_move
+    kill thread_trajectory
+    kill thread_script_commands
     stopj(STOPJ_ACCELERATION)
     halt
   end