Docs: prepare direct translation of node names in Texinfo sources

Docs: prepare direct translation of node names in Texinfo sources

- fix nits in texi-gettext.py,
- update tely-gettext.py and extract_texi_filenames.py,
- use --master-map-file option of extract_texi_filenames.py added in
this commit,
- remove embarrassing node translations in French and Spanish doc POs.

All these updates have been tested with a few makefiles hooks (not
included in this commit), an apparently successful execution of
tely-gettext.py on all translations followed by 'make doc' and a quick
check of HTML and PDF output.