replace output format customization with TEXINFO_OUTPUT_FORMAT

[Patrice Dumas]

Hello,

I am about to add TEXINFO_OUTPUT_FORMAT also as a customization variable
with the same effect as setting TEXINFO_OUTPUT_FORMAT in the
environment.  That allows to set all the more or less exotic formats by
setting TEXINFO_OUTPUT_FORMAT to the format name.  So, DEBUGCOUNT,
DEBUGTREE, PLAINTEXINFO, RAWTEXT, TEXTCONTENT, TEXINFOSXML could be
removed.  For example -c TEXTCONTENT=1 would be come 
TEXINFO_OUTPUT_FORMAT=textcontent.

I would be in favor of that, would you agree?

-- 
Pat