libutil: Document hacks and problems around Pos class

This should provide context for follow-up commits in
the patch series.

(cherry picked from commit bf12aedf2e)

src/libutil/position.hh

@@ -50,6 +50,7 @@ struct Pos
 
     explicit operator bool() const { return line > 0; }
 
+    /* TODO: Why std::shared_ptr<Pos> and not std::shared_ptr<const Pos>? */
     operator std::shared_ptr<Pos>() const;
 
     /**