devstat(9): Constify function parameters that can be const

devstat(9): Constify function parameters that can be const

No functional change.

When attempting to document the changed argument types in devstat.9, I
discovered the 20 year old manual page severely mismatched reality even
prior to my simple change.  So I took a first cut pass cleaning that up to
match reality.  I'm sure I've missed some things; the goal was just to leave
it better than when I started.

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