need to modify registered ActivationDesc and ActivationGroupDesc

Activatable objects and groups need to be able to change the contents of the
registered descriptor that is used to activate the object or group. If some
of the information in the object's descriptor changes, such as the classname
of the implementation of the object changes, or the bootstrap information that
the object needs during startup changes, or the codebase URL for the object
changes, the activation descriptor information needs to be updated. This
issue came up in our discussions with customers at JavaOne.

We propose adding two new methods to java.rmi.activation.ActivationSystem
to update the contents of the ActivationDesc and ActivationGroupDesc
registered for an object and group respectively.