INT 2F - DOS 3.0+ internal - FIND UNREFERENCED DISK BUFFER

	AX = 1210h
	DS:DI -> first disk buffer to check
Return: ZF clear if found
	    DS:DI -> first unreferenced disk buffer
	ZF set if not found
Note:	in DOS 5+, this has become essentially a NOP, invoking the same code
	  used by AX=1224h (SHARING DELAY)
SeeAlso: AX=120Eh