This adds the modifiers CONVERT and NOCOVERT to the HI units. When
enabled for a unit, 1822 messages will transparently be converted.
IMP-to-host messages are converted from the old, short (32-bit) format
to the new, long (96-bit) format. Host-to-IMP messages are converted
in the other direction.
The motivation for this feature, is that the currently running IMP
software is from 1974 and only supports short leaders. Some operating
systems are from a later era, and only support long leaders.
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