generating Info encoded in UTF-8 only

Hello,

I propose to generate Info encoded in UTF-8 only from texi2any.  This
has two advantages
* simplify the implementation in C (in Perl still need to encode/decode)
* non-ASCII cross-references between manuals are correct in Info reader

I do not see any disadvantage.

For the plaintext output, it could be simpler if we did the same, but
encoding the output would probably be easy if we make the change for
Info first.

-- 
Pat