kboot: For hostfs, return better errors from read, where possible.

[imp]

Summary

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.

Sponsored by: Netflix

[imp]

rebase / refactor / redo

[kevans]

stand/kboot/host_syscall.h

211

[imp]

fix spelling

[brooks]

I did find myself wondering if host_to_stand_errno should assert that e <= 0 since it's a relatively common error to accidentally use positive errors when modifying linux code.

[imp]

In D38265#871758, @brooks wrote:

I did find myself wondering if host_to_stand_errno should assert that e <= 0 since it's a relatively common error to accidentally use positive errors when modifying linux code.

I had similar thoughts early on, but this wound up being a #define... I'll post a followup review.