Use bit 23 of the device minor to indicate a control mode device. A

control mode device can be opened but does not permit any i/o.

Currently no such devices are created by MAKEDEV. However the previous
code used bits 0 and 1 of the minor being set to indicate a control
device. This conflicted with other uses of device minors to indicate
things like rewind and eject. This behaviour would result in an open
of a newly loaded tape drive failing until the tape was successfully
mounted as a side effect of another command like 'mt status'.

ok beck@

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