Redmine 6523: Avoid read/write access to Setuid storage

We only ever add (prepend) to this list; we save it and restore it; so if a file is recorded as setxid ever, it'll always be so.  I see no way to clear a file from this state.  Given that setxid should be used with caution, I can imagine an admin might want to retract it from a file sometimes.  We seem to be leaving no way to make that stick ...