doc: declare encoding for texi2html.

It adds the following header to the HTML files:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
which helps some browsers to display correctly the few non-ASCII
characters in the doc.

doc/t2h.init

@@ -158,6 +158,9 @@ $AFTER_BODY_OPEN
 EOT
 }
 
+# declare encoding in header
+$IN_ENCODING = $ENCODING = "utf-8";
+
 # no navigation elements
 $SECTION_NAVIGATION = 0;
 # the same for texi2html 5.0