Add xmlXPathSetContextNode and xmlXPathNodeEval

This patch adds xmlXPathSetContextNode and xmlXPathNodeEval,
which make it easier to evaluation XPath expressions with a
context node other than the document root without poking about
inside the internals of the context.

This patch is compile-tested only, and is my first libxml2
contribution, so please go easy.

Signed-off-by: Alex Bligh <alex@alex.org.uk>

include/libxml/xpath.h

@@ -508,6 +508,13 @@ XMLPUBFUN int XMLCALL
  */
 XMLPUBFUN long XMLCALL
 		    xmlXPathOrderDocElems	(xmlDocPtr doc);
+XMLPUBFUN int XMLCALL
+		    xmlXPathSetContextNode	(xmlNodePtr node,
+						 xmlXPathContextPtr ctx);
+XMLPUBFUN xmlXPathObjectPtr XMLCALL
+		    xmlXPathNodeEval		(xmlNodePtr node,
+						 const xmlChar *str,
+						 xmlXPathContextPtr ctx);
 XMLPUBFUN xmlXPathObjectPtr XMLCALL
 		    xmlXPathEval		(const xmlChar *str,
 						 xmlXPathContextPtr ctx);