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.