Set broadcast mode (INT 21h, DEh - Novell NetWare 4.0)

        Entry:
          AH = DEh
          AL = broadcast mode
                 00h  Receive console and workstation broadcasts
                 01h  Receive console broadcasts only
                 02h  Receive no broadcasts
                 03h  Store all broadcasts for retrieval
                 04h  Get broadcast mode
                 05h  Disable shell timer interrupt checks
                 06h  Enable shell timer interrupt checks
        Return:
          AL = old broadcast mode