Extensibility changes

Reflexive extension functions must now use the "strict" URI format (for example "java:java.util.Date") rather than the "liberal" format (which allows for example "http://my.com/extensions/java.util.Date"), unless the configuration property ALLOW_OLD_JAVA_URI_FORMAT is set. This change was documented for 9.2 but not implemented (the default value for the flag was to allow the old URI syntax). Note that because dynamic extension functions require at least Saxon-PE, this flag is not recognized in Saxon-HE.