This routine performs a SCSI Position to Element command. This
    command positions the trasport to the specified destination
    element. It is used by mrd_position(3mrd) function.

    The robot_info argument is the address of a robot_info_t that has
    been opened by mrd_startup(3mrd). This allows multiple position
    commands (and other commands) to be executed without having to
    repeat the startup for each command.

    The transport address is the absolute address of the transport
    element to be used for the command. Many medium changers allow
    the use of address zero (0) as the default transport, but some
    may require a transport address that is valid for the medium
    changer. For single transport medium changers, the transport
    base address of the robot_info_t structure, transport_start is a
    suitable address.

    The destination address is the absolute addresses to be used as
    the destination for the command . The absolute address can be
    calculated from a zero relative address by adding it to the base
    address for the element type. The routine makes no checks for
    the validity of the address, relying on the medium changer to do
    this.

    A invert value of one (1) can be used on medium changers that
    support inverting the transport, when this is desired; an optical
    drive with two sided media. Otherwise a value of zero should be
    used.

    This routine uses the dev_status_t structure for handing errors.
    The dev_status_t structure includes the code, os_status, and SCSI
    error fields. The following describes how to decode errors with
    the dev_status_t structure.

    SCSI Errors

    SCSI errors are indicated when the value of the valid field of
    the SCSI error is not equal to 0. The key, asc, and ascq fields
    provide additional information to help determine the cause of the
    error.

    The code usually maps the Additional Sense Code and Additional
    Sense Code Qualifier (ASC/ASCQ) values to an MRD error. The asc
    and ascq values are copied from the request sense data returned
    by the target.

    The Additional Sense Code (asc) indicates further information
    related to the error or exception condition reported in the sense
    key field. The Additional Sense Code Qualifier (ascq) indicates
    detailed information related to the additional sense code. For
    more information, consult the SCSI-2 Specification.

    Operating System Errors

    Operating system errors are indicated when the value of the valid
    field of the SCSI error is equal to 0 and the value of the os_
    status field is not equal to 0. This result is most likely caused
    by an operating system error, and probably has a mapped error in
    MRD.

    MRD Errors

    MRD errors are indicated when the value of the os_status field is
    0, and the value of the valid field of the SCSI error is 0. This
    result is most likely caused when MRD encounters its own failure.