INT 15 - Stac STACKER - INSTALLATION CHECK                                      
	AX = B001h
	CX = 5354h ('ST')
	DX = 4143h ('AC')
Return: AX = 4F4Bh ('OK') if installed
Note:	this function is encapsulated in Stac's STACKER.LIB C library; it is
	  called by Novell DOS 7+ NWCACHE v1.01, which makes use of this
	  library