Given a robot_info_t structure and absolute element address,
    this routine figures out the corresponding element type and zero
    relative address. The relative address is formatted into the
    space provided by result and the element type is returned.

    A valid robot_info_t structure can be obtained by using mrd_
    startup(3mrd) or mrd_show(3mrd) to open the robot and fill in the
    robot_info_t structure.

    The SCSI-2 specification allows an absolute address of zero (0)
    to refer to a default transport, when a medium-changer has more
    than one. When handed zero as the absolute address, this routine
    will reflect this convention even if the particular medium-
    changer doesn't.