Fix issue 1819: add support for tagging properties as advanced.

It is now possible to tag a property as advanced. This is achieved by adding the "advanced" attribute to XML property definitions. For example:

   <adm:property name="java-implementation-class" mandatory="true" advanced="true">
     ...
   </adm:property>

Advanced properties must be either option or, if they are mandatory, have default values (defined or inherited). This constraint is enforced except for abstract property definitions where sub-definitions can override the property and give it a default (this is the case for java implementation class properties).

Dsconfig has been modified so that get-xxx-prop and list-properties sub-commands only display advanced properties if the user provides the "--advanced" command line option. There will be similar support for set-xxx-prop and create-xxx sub-commands once they have a fully functional interactive mode (issue 1831).

This change just adds support for advanced properties. A subsequent change (for issue 1829) will designate which properties are advanced.