Get shift status

        Entry:
          AH = 02h
        Return:
          AL = shift status bits

            1xxx xxxx  Right shift key depressed
            x1xx xxxx  Left shift key depressed
            xx1x xxxx  CTRL depressed
            xxx1 xxxx  ALT depressed
            xxxx 1xxx  SCROLL LOCK active
            xxxx x1xx  NUM LOCK active
            xxxx xx1x  CAPS LOCK active
            xxxx xxx1  INSERT state active