Convert HTML pages into Unicode

[hpreusse]

Our nice quality checker lintian reports that some (generated) HTML files contain national encoding, many files for the french part of the docs. Indeed, "file" prints something like

hille@debian-amd64-sid:~/devel/zzz_empty/MWE$ file GoingfurtherOthertools.html
GoingfurtherOthertools.html: HTML document, Non-ISO extended-ASCII text

According to the tex4ht people, one needs to specify to create utf-8 at the command line or one should use make4ht instead. I tested both, I can confirm that both methods works; in addition the tex4ht.env needs to be removed.

Based on this I created a patch (the alternative option make4ht is contained but commented). If you need me to create a pull request for this patch, call back.