INT 21 - DOS 4.x only - TRUNCATE OPEN FILE TO ZERO LENGTH
        AX = 5704h
        BX = file handle (only 0000h through 0004h valid)
        DS:SI -> ??? passed through to INT 2F/AX=112Dh
        ES:DI -> result buffer
        CX = size of result buffer
Return: CX = size of returned data
        ES:DI -> zero word (DOS 4.0) if CX >= 2 on entry
SeeAlso: AX=5703h,INT 2F/AX=112Dh