NAME
	TD_CHANGENUM -- return the current value of the disk-change counter.

   FUNCTION
	This command returns the current value of the disk-change counter (as
	used by the enhanced commands). The disk change counter is incremented
	each time a disk is inserted or removed from the trackdisk unit.

   IO REQUEST INPUT
	io_Device	preset by the call to OpenDevice()
	io_Unit		preset by the call to OpenDevice()
	io_Command	TD_CHANGENUM
	io_Flags	0 or IOF_QUICK

   IO REQUEST RESULT
	io_Error - 0 for success, or an error code as defined in
	           <devices/trackdisk.h>
	io_Actual - if io_Error is 0, this contains the current value of the
		    disk-change counter.