wrong namespace for id in the ITS module

The W3C its: namespace reuses xml:id on the elements it defines
Our its.xsd introduces an its:id of the type xs:id
I am afraid there is no such attribute in the W3C namespace because it
actually reuses the xml:id.

Thus, the xsd needs to change the its:id to xml:id and the prose spec needs
to make clear in 5.9.12 and 5.9.12.4 that the module uses xml:id and not
its:id (or even itsm:id that it had been using in csprd01). Both are of the
xs;id type, just that we don't control the its: namespace so that we are
not free to declare it where it didn't exist..