configitems: get default values from the central registry when available We do not have any registered config yet, but we are now ready to use them. For now we ignore this feature for config access with "alternates". On the long run, we expect alternates to be handled as "aliases" by the config item themself.

configitems: introduce a central registry for config option We now have the appropriate infrastructure to register config items. Usage will added in the next changeset.