Duncan Coutts authored Feb 16, 2016 and Mikhail Glushenkov committed Mar 17, 2016

The existing approach has been to parse top level fields and then
separately parse each top level section, and similarly for pretty
printing. A better approach is to follow the pattern for fields and have
a section description. Then we just parse or print fields+sections in
one call. And as a bonus, secitons can have subsections (and could even
do so recursively).

This patch just adds the new functionality. No existing config parsing
is switched over.

(cherry picked from commit 2f743b6d)