INT 16 U - PC Tools v7+ DESKTOP - OPEN \DESK.OVL FILE AND SEEK TO OVERLAY
        AX = FF98h
        DX = byte offset in file of overlay header
Return: BX = file handle for DESK.OVL file
Desc:   open the DESK.OVL file, seek to the specified offset, read in the
          overlay header, and seek to the offset specified by the header

Format of overlay header:
Offset  Size    Description
 00h 12 BYTEs   NUL-padded ASCII overlay filename
 0Ch    DWORD   offset within DESK.OVL file of actual overlay

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