INT 21 - Phar Lap 386/DOS-Ext VMM v3.0 - GET OFFSET OF .EXP FILE HEADER

	AX = 2539h
	BX = MS-DOS file handle for open file
Return: CF clear if successful
	    EAX = offset of .EXP header in file
	CF set on error
	    EAX = error code (02h file error)
	    EBX = file error code
		02h DOS error seeking
		03h DOS error reading
		04h invalid file type
		05h invalid file format
	    ECX = DOS error code if EBX=02h or 03h
	current file position in file modified