texi2html: Create tab-separated file containing the NODE<=>FILENAME map

texi2html: Create tab-separated file containing the NODE<=>FILENAME map

For each main html file, I now write out the map of filename<=>nodename
associations to the file basename_xref.map (where basename is e.g. index).
This file contains tab-separated lines of the form

NODENAME\tFILENAME\tANCHOR_IN_FILE

In the future, this association will be used by the other files to
properly resolve external references. Of course, this will require processing
each texi file twice...