Fix xpath() to return namespace definitions

Hi all,

While developing some XML processing queries, i stumbled on an old bug
mentioned in http://wiki.postgresql.org/wiki/Todo#XML: Fix Nested or
repeated xpath() that apparently mess up namespaces.

Source of the bug is that libxml2's xmlNodeDump won't output XML namespace
definitions that declared in the node's parents. As per
https://bug639036.bugzilla-attachments.gnome.org/attachment.cgi?id=177858,
the behavior is intentional.

This patch uses function xmlCopyNode that copies a node, including its
namespace definitions as required (without unused namespace in the node or
its children). When the copy dumped, the resulting XML is complete with its
namespaces. Calling xmlCopyNode will need additional memory to execute, but
reimplementing its routine to handle namespace definition will introduce
much complexity to the code.

Note: This is my very first postgresql patch.

--
Ali Akbar