INT 2F U - DOS 3.3+ internal - OPEN FILE
        AX = 1226h
        CL = access mode
        DS:DX -> ASCIZ filename
        SS = DOS DS (must be using a DOS internal stack)
Return: CF set on error
            AL = error code (see INT 21/AH=59h)
        CF clear if successful
            AX = file handle
Notes:  can only be called from within DOS
        equivalent to INT 21/AH=3Dh
SeeAlso: INT 21/AH=3Dh