config: write out section headers with subsections correctly

config: write out section headers with subsections correctly

write_section() mistakenly treated is input as the whole variable name
instead of simply the section (and possibly subsection) and would
confuse "section.subsection" as a section plus variable name and
produce a wrong section header.

Fix this and include a test for writing "section.subsection.var" and
reading it from the file.