texi2html: Fix issues with snippets page, fix coding errors

texi2html: Fix issues with snippets page, fix coding errors

-) use eq instead of == for string comparisons
-) unnumberedsec (snippets!) don't need an entry in the map file (they should
   have one, but I don't write out a warning, because with snippets it just
   doesn't work otherwise: A snippet can be in multiple sections, so I cannot
   use the snippet name to look up the proper file in the hash. For this
   reason, I don't handle snippets snippets at all with my
   extract_texi_filename.py script and instead let the .init file fall back
   to the default of putting all unnumbered sections into the previous
   file).