config: doveconf - If filter_name exists, show filter_name_key under filter_name...

config: doveconf - If filter_name exists, show filter_name_key under filter_name { key }

This isn't done for default settings, because it's too much effort to
implement. Also this isn't visible for the usual "doveconf -n" or
"doveconf -N" outputs. Normally people aren't looking at just "doveconf" or
"doveconf -d" output, so it likely doesn't matter.