score: Fix _Thread_Cancel()

The _Thread_Cancel() (in contrast to _Thread_Restart() which used a
similar code block) may have produced ready threads with an active timer
in case the thread to cancel had its thread life protection enabled. The
problem was this code block:

    Priority_Control priority;

    _Thread_Add_life_change_request( the_thread );

    if ( _Thread_Is_life_change_allowed( previous ) ) {
      _Thread_State_release( the_thread, &lock_context );

      _Thread_queue_Extract_with_proxy( the_thread );
      _Thread_Timer_remove( the_thread );
    } else {
      _Thread_Clear_state_locked( the_thread, STATES_SUSPENDED );
      _Thread_State_release( the_thread, &lock_context );
    }

    priority = _Thread_Get_priority( executing );
    _Thread_Raise_real_priority( the_thread, priority );
    _Thread_Remove_life_change_request( the_thread );

The life change request should only be added/removed if a life change is
allowed (see _Thread_Restart()).  Add _Thread_Try_life_change_request()
and use it in _Thread_Cancel() and _Thread_Restart().

Close #4435.

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