Join thread_move instead of killing it (UniversalRobots#349)

Once we change the operational mode to STOPPED in the main thread, the
move thread should stop by itself, so we can wait for it to join.

Author: urfeex

resources/external_control.urscript

@@ -806,7 +806,7 @@ while control_mode > MODE_STOPPED:
 end
 
 textmsg("ExternalControl: Stopping communication and control")
-kill thread_move
+join thread_move
 kill thread_trajectory
 kill thread_script_commands
 stopj(STOPJ_ACCELERATION)