Permit tables of contents in HTML single-file mode.

Patch due to J. Lewis Muir, who points out that the HTML backend's
current policy of _always_ disabling the TOC in single-file mode is
excessively harsh: in a long or formal enough document, you might
still want a TOC to make navigating around within the file easier,
even if it's not necessary to use it to get between multiple files.

So this change removes the unconditional prohibition against TOCs in
single-file documents, but they're still disabled by default, because
a single file counts as a leaf file and the existing default settings
disable TOCs in leaf files anyway. So if you do want a TOC in a single
file, you can reconfigure 'html-leaf-contains-contents' to true.


git-svn-id: svn://svn.tartarus.org/sgt@10212 cda61777-01e9-0310-a592-d414129be87e