Change Texinfo output to UTF-8

Change Texinfo output to UTF-8

Since the whole documentation tool chain is now UTF-8 and there is an
increasing number of non-ISO-8859-1 characters in the text, keeping
the Texinfo output in ISO 8859-1 just creates unnecessary
complications.  Depending on the platform, there are conversion
failures and thus build failures, or weirdly converted characters.  By
changing the output to UTF-8, the whole encoding conversion business
is sidestepped.