According to the CSL manual, the default entry spacing is 1.
We were treating it as 0.
T.P.Citeproc: always include an entry-spacing attribute
in the Div if the bibliography element contains an entry-spacing
attribute (previously we omitted it when it was 0).
LaTeX writer: use entry spacing 1 if no entry-spacing
attribute is present.
Update tests.
See #9058.
Previously we used Setext (underlined) headings by default.
The default is now ATX (`##` style).
* Add the `--markdown-headings=atx|setext` option.
* Deprecate `--atx-headers`.
* Add constructor 'ATXHeadingInLHS` constructor to `LogMessage` [API change].
* Support `markdown-headings` in defaults files.
* Document new options in MANUAL.
Closes#6662.