use all files in some directories for htmlxref

[Patrice Dumas]

Hello,

I think that it would be better to read all the files in some
directories (only files ending with .conf) to find the htmlxref entries.

I think that it is better to read only one file htmlxref.cnf (or
HTMLXREF_FILE In the current directory, in the manual directory and in
the user HOME.

But in $datadir/texinfo, $sysconfdir/texinfo, $XDG_DATA_DIRS,
$XDG_CONFIG_DIRS, /usr/share/texinfo and /usr/local/share/texinfo, it
would be better, in my opinion, to have a directory named htmlxref/
(or htmlxref.d) and merge all the files ending with .conf.  We could
also read htmlxref.cnf (or HTMLXREF_FILE) directly in $dir/texinfo/ for
backward compatibility.

The idea is to make it simpler
* to have diverse groups add their list of htmlxref entries.  This could be
  a step towards what Eli proposed.
* to have GNU/Linux distributions add htmlxref entries.
* for other software packages to add their lists (texi2html if it was still
  maintained, or other software that generate online manuals that can be
  targets of Texinfo cross-references).

We could split the htmlxref.cnf we maintain in GNU/nonGNU, over GNU we
would be authoritative, but not for other entries.

The only drawback I see is that it becomes easier to put a wrong entry.
Maybe we could add a customization variable to report double entries to
mitigate that issue.

What do you think?

-- 
Pat