build-namelist improvement, automatically add $nl_flags to add_defaults calls

It's easy to forget to add needed attributes to both the namelist_defaults XML file as well as the perl code. This happens to all of us, and we struggle with why the namelist isn't working as expected until we remember this key detail.

If we make $nl_flags a required argument to add_default calls, ALL of the flags in $nl_flags could be automatically used as attributes. Which means it would be unlikely you'd need to update the perl code with many changes to namelist options.

This would be a time savings for development.