hierarchicalize:  Do not number section with class "unnumbered".

Unnumbered sections get [] for their section number.
So far only the HTML writer has been adjusted to be sensitive
to this.

If we keep this change, all the writers will need to be changed
either (a) to directly check for the "unnumbered" class, if they
do section numbering themselves, or (b) to check for a null section
number, if they use hierarchicalize.