Clean up .xref-map files generation

Clean up .xref-map files generation

- write these files in (top-build-dir)/out/xref-maps/, with language
used as name suffix if different from English: this is cleaner than
writing them in out-www subdirectories and creating symlinks, mainly
because it avoids hardcoding manual names in a bunch of makefiles;

- buildscripts/extract_texi_filenames.py: write "@documentlanguage" as
output file suffix if not "en", create output directory if necessary,
and fix minor nitpicks;

- remove all hardcoded manual names in makefiles.