XML: add namespace-aware node accessors

XML: add namespace-aware node accessors

The existing $attr$name and $tag$name accessors compare local names
only, so x:ID and ID, or x:Issuer and Issuer, are indistinguishable.

The new XMLNode methods getAttributeNS(namespaceURI, localName) and
getChildNS(namespaceURI, localName) match both the namespace URI and
the local name.