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);

xpath.c

@@ -15078,6 +15078,49 @@ xmlXPathEval(const xmlChar *str, xmlXPathContextPtr ctx) {
     return(res);
 }
 
+/**
+ * xmlXPathSetContextNode:
+ * @node: the node to to use as the context node
+ * @ctx:  the XPath context
+ *
+ * Sets 'node' as the context node. The node must be in the same
+ * document as that associated with the context.
+ *
+ * Returns -1 in case of error or 0 if successful
+ */
+int
+xmlXPathSetContextNode(xmlNodePtr node, xmlXPathContextPtr ctx) {
+    if ((node == NULL) || (ctx == NULL))
+        return(-1);
+
+    if (node->doc == ctx->doc) {
+        ctx->node = node;
+	return(0);
+    }
+    return(-1);
+}
+
+/**
+ * xmlXPathNodeEval:
+ * @node: the node to to use as the context node
+ * @str:  the XPath expression
+ * @ctx:  the XPath context
+ *
+ * Evaluate the XPath Location Path in the given context. The node 'node'
+ * is set as the context node. The context node is not restored.
+ *
+ * Returns the xmlXPathObjectPtr resulting from the evaluation or NULL.
+ *         the caller has to free the object.
+ */
+xmlXPathObjectPtr
+xmlXPathNodeEval(xmlNodePtr node, const xmlChar *str, xmlXPathContextPtr ctx) {
+    if (str == NULL)
+        return(NULL);
+    if (xmlXPathSetContextNode(node, ctx) < 0)
+        return(NULL);
+    return(xmlXPathEval(str, ctx));
+}
+
 /**
  * xmlXPathEvalExpression:
  * @str:  the XPath expression