resolvectl: consolidate JSON and regular status output paths

resolvectl: consolidate JSON and regular status output paths

Now that varlink is always used for status output, consolidate the JSON
and regular status code paths. Add status_full(), which is the single
point that calls DumpDNSConfiguration, filters based on link names and
mode if needed, and prints the output in the appropriate format.

This simplifies status_ifindex(), and removes the need for a separate
status_json().