man: add bootchart.conf to directives index
make-directive-index: remove duplicated list of sections
make-directive-index: count how many directives and pages are shown
make-directive-index: link to systemd options
man: add links to directive index to see-alsos

systemd.directives(5) is renamed to systemd.directives(7).
Section 7 is "Miscellaneous".
man: generate xml not html for index

This way we also get a man page. The output is not as polished.
I hope that it doesn't matter too much.

index.html is not generated now, the page is called
systemd.index.html. If necessary, an install hook should be added.
make-man-index: work around UnicodeDecodeError