texi2html: Create map for snippets, link other map files, rename to .xref-map

texi2html: Create map for snippets, link other map files, rename to .xref-map

-) Renamed the map files to .xref-map, so that they are indicated by extension
   only (instead of _xref.map)
-) Add map file as dependency for the snippets page, so that it will be
   generated automatically
-) Linked the map file for the snippets in Documentation/user/out-www and the
   map files from Documentation/user in input/lsr/out-www, so that cross
   references between those manuals work correctly, too.