texi2html: Fix unnumbered sections (IR), node anchors (big-page)

texi2html: Fix unnumbered sections (IR), node anchors (big-page)

-) unnumbered sections (except at top-level) now completely ignore the
   file name from the .xref-map file and are instead simply put into
   the same file as the previous numbered section (this fixes the
   problem with user/internal property pages for the interfaces in the
   IR)
-) add a lilypond_node_file_name function, which modifies also the
   anchors for the nodes (split_at_numbered_sections already
   modified the anchors for sections).