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>