specification: Add missing catalogue for 2007-06 schema
specification: Add missing xml.xsd entity to external catalogue
cpp: xerces: Add catalogue registration to EntityResolver
cpp: xerces: Minor wrapper improvements
cpp: xerces: Add string comparison operators
cpp: xerces: Recursively process XML catalogues
cpp: xerces: Reduce catalogue initialisation overhead

Rather than statting every schema file to make sure it
exists before adding it to the entity map, just add it
unconditionally.  If the assumption it exists is wrong,
it will just require fetching the slow way.  The only
other failure cases are if this means it won't try an
alternate but valid version of the uri which is
registered afterward.  However, since this would
indicate a bug in our catalogues and will be very
uncommon, instead cater for the common case where the
installation is not broken.