INT 16 U - PCIBM PC Tools v7+ DESKTOP - OPEN \DESK.OVL FILE AND SEEK TO OVERLAY
	AX = FF98h
	DX = byte offset in file of overlay header (see #00619)
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 PCIBM PC Tools DESKTOP overlay header:
Offset	Size	Description	(Table 00619)
 00h 12 BYTEs	NUL-padded ASCII overlay filename
 0Ch	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	offset within DESK.OVL file of actual overlay