object: subdisks, plexes and volumes. The encoding for plexes and
subdisks no longer reflects the object to which they belong. The
super devices are high-order volume numbers. This gives vastly more
potential volumes (4 million instead of 256).
As a result of the minor number changes, split out the superdevice
handling into a separate function, vinum_super_ioctl. This was most
of the code of vinumioctl.
attachobject: Improve error checking.
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