INT 21 - DOS 2+ - "EXIT" - TERMINATE WITH RETURN CODE
        AH = 4Ch
        AL = return code
Return: never returns
Notes:  unless the process is its own parent (see AH=26h, offset 16h in PSP),
          all open files are closed and all memory belonging to the process
          is freed
        all network file locks should be removed before calling this function
SeeAlso: AH=26h,AH=4Bh,AH=4Dh,INT 15/AH=12h/BH=02h,INT 20,INT 22