Don't assume the name of the table of contents will be contents.html;

Don't assume the name of the table of contents will be contents.html;
that won't be right when mkhowto is called with --numeric (as it is
for the Python Tutorial).  Save the actual name we use when we build
the table of contents, then use that in the page head metadata.  The
node2label.pl script will munge this properly when --numeric isn't
given, so this works in both cases.
Closes SF bug #761830.