NAME
	TD_CHANGESTATE -- check if a disk is currently in a drive.

   FUNCTION
	This command checks to see if there is currently a disk in a drive.

   IO REQUEST INPUT
	io_Device	preset by the call to OpenDevice()
	io_Unit		preset by the call to OpenDevice()
	io_Command	TD_CHANGESTATE
	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 tells you whether a disk is in
		    the drive. 0 means there is a disk, while anything else
		    indicates there is no disk.