[tex4ht] consolidating all tex4ht configuration files into one standard file?

[Karl Berry]

    The only thing I know about this, is that when using make4ht, then
    the file texmf-dist/tex4ht/base/unix/tex4ht.env is not used/read
    any more as would be the case when using htlatex directly. Which
    is a good thing, 

Or, it's a bad thing, because now everything that was configured in
tex4ht.env must also be configured in a second place.  That sounds
impossible to maintain.  I'm sure Michal had good reasons, though.
Michal?

    as tex4ht.env is a very confusing configuration file.

Agreed. -k