Enable/disable OS function set functions

        Entry:
          AH = 5Dh
          AL = subfunction
                00h enable OS Function Set
                01h disable OS Function Set
                02h return access key (resets memory manager, returns
                    access key at next invocation)
          BX,CX = access key returned by first invocation
        Return:
          BX,CX = access key, returned only on first invocation of
                  function
          AH    = status
            00h successful
            80h internal error
            81h hardware malfunction
            84h undefined function requested
            8Fh undefined subfunction
            A4h operating system denied access