Translate the Linux error return from read to a FreeBSD errno. We use a
simplified translation: 1-34 are the same between the systems, so any of
those will be returned directly. All other errno map to EINVAL. This
will suffice for some code that reads /dev/mem in producing the right
diagnostic.
A fully generalized version is much harder. Linux has a number of errno
that don't translate well and has architecture dependent
encodings. Avoid this mess with a simple macro for now. Add comment
explaining why we use the simple method we do.
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