Replace unknown XML entities with their numerical counterpart.

Replace unknown XML entities with their numerical counterpart.

Texinfo seems to produce XML entities (such as ©) which
are not valid (although they are valid for HTML).  We work
around such problems by post-processing with sed. Here, we
replace these unknown entities with their numeric counterparts.
Such is the advice given at
http://www.dwheeler.com/essays/quotes-in-html.html