Change html manual encoding to UTF-8.

This works around GitHub's broken automatic reformatting from ISO-8859-1
to UTF-8 when serving static html.

Remove <parameter/> from e.g. <function>malloc<parameter/></function>,
add a custom template that does not append parentheses, and manually
specify them, e.g. <function>malloc()</function>.  This works around
apparently broken XSL formatting that causes <code/> to be emitted in
html (rather than <code></code>, or better yet, nothing).

doc/html.xsl.in

@@ -1,4 +1,5 @@
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
   <xsl:import href="@XSLROOT@/html/docbook.xsl"/>
   <xsl:import href="@abs_srcroot@doc/stylesheet.xsl"/>
+  <xsl:output method="xml" encoding="utf-8"/>
 </xsl:stylesheet>