Set oem int 21 handler

        Entry:

          AH = F8h
          DS:DX -> OEM INT 21 handler for functions F9h to FFh
                   FFFFh:FFFFh resets to original handlers

Notes:  Calls to AH=F9h through AH=FFH will return CF set and AX=1 (invalid
        function) if no handler set
        Handler is called with all registers exactly as set by caller, and
        should exit with IRET