Update xml

Along with the third-party libxml updates, this exposes two xpath functions for general-purpose use:

revXMLEvaluateXPath(pDocID, pXpathExpression [, delimiter])
revXMLDataFromXPathQuery(pDocID, pXpathExpression [, delimiter])

I'm not yet up on how to get these into the doc files.
See http://forums.runrev.com/viewtopic.php?f=66&t=15624&start=15#p79612