Create/open/delete file

        Entry:
          AX = FFD4h
          BH = 3Ch create file
                  CX = file attributes
               3Dh open file
               41h delete file
          BL = access mode
               00h read only
               01h write only
               02h read/write
          DS:SI -> ASCIZ filename

        Return:
          BX = file handle
               0000h on error