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

   FUNCTION
       This command returns the current value of the disk-change counter
       The disk change counter is incremented each time a disk is inserted
       or removed from the cd unit.

   IO REQUEST INPUT
       io_Device       preset by the call to opendevice()
       io_Unit         preset by the call to opendevice()
       io_Command      CD_CHANGENUM

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