INT 21,F8 - Set OEM Int 21 Handler (functions F9-FF, Undocumented)

	AH = F8
	DS:DX = pointer to OEM handler for INT 21H calls F9 through FF
	      = FFFF:FFFF to reset to original handlers

	- DOS is set up to allow ONE handler for all 7 function calls.
	  Any call to these handlers will set the CF and AX
	  will contain 1 if they are not initialized.	The handling
	  routine is passed all registers just as the user set them.
	  The handler should exit with an IRET.