INT 6B - NPC NCSI EXTENDED SERIAL I/O - DISCONNECT A VIRTUAL CIRCUIT            
	AH = 17h
	AL = virtual circuit number
Return: CF clear if successful
	CF set on error
	???
Note:	this function is also supported by TelAPI, which always returns CF
	  clear and AL=00h
SeeAlso: AH=11h,AH=16h