cwfs had an issue with iounit negotiation as a result
of the conversion to 9p2000 -- with the move to variable
size messages, the fixed message overhead decreased, but
the advertised message size was still adding the old
fixed overhead.
This meant that if the kernel negotiated the maximum io
size, cwfs would negotiate something larger than it
supported, and would hang up when an io of that size
was made.
In addition, the size of messages was stored in a short,
which means that negotiating an iounit larger than 16384
bytes would overflow the message count, and cause things
to fall over.
Finally, whle we're here, we clean up some duplicated
and unused constants.
4eeefed7b0