Section levels

While trying to get the section levels to have the
right size in print and HTML at the same time I found
out that the code really needed some structural change
because the heading levels need to be computed
differently in chunked and non-chunked mode. Plus,
some things aren't chunked even in chunked mode, like
refentries, so the simple "add 1" games don't really
work correctly.

I find this new code to be a lot better structured, and
it also fixes the "hackish" treatment of the renderas
attributes (using the 5th character of the attribute
value directly as section level, which really doesn't
work if you believe the previous paragraph).