manual / manpages: Adjust option filter filtering, move from C++ to Nix

Behavior change:

Before we only showed uption if the command-specific options were
non-empty. But that is somewhat odd since we also show common options.
Now, we do everything based on the union of both sorts of options (with
hidden-categories filtered, as before).

Implementation change:

The JSON dumping once again includes all options; the filtering of
hidden categories is done in the Nix instead. This is better separation
of "content" vs "presentation", and prepare the way for the HTML manual
vs manpages / `--help` doing different things.

doc/manual/generate-manpage.nix

@@ -75,10 +75,14 @@ let
         (details ? doc)
         (replaceStrings ["@stores@"] [storeDocs] details.doc);
 
-      maybeOptions = optionalString (details.flags != {}) ''
+      maybeOptions = let
+        allVisibleOptions = filterAttrs
+          (_: o: ! o.hiddenCategory)
+          (details.flags // toplevel.flags);
+      in optionalString (allVisibleOptions != {}) ''
         # Options
 
-        ${showOptions (details.flags // toplevel.flags)}
+        ${showOptions allVisibleOptions}
 
         > **Note**
         >