[6.x.x] xmldb:xupdate#3 function applies an XUpdate to a single document

[adamretter]

Backport of #131

Previously an XUpdate could only be applied from an XPath function call to an entire Collection, we add the function:

xmldb:update($collection-uri as xs:string, $document-uri as xs:string, $modfications as node()) as xs:integer

to apply XUpdate to a single document.