Support initial values for system-category properties on config nodes

There is sometimes a need for a property to be initialized when a node is first created, but for that property to afterwards be managed by the runtime environment. The password for the default "admin" user is the most critical use case, though this might also apply elsewhere. Once the value is set, it should not be kept in sync with the config definition at bootstrap, should not be exported by the upgrade verifier, etc.

For the initial implementation, it is acceptable for auto-export to ignore changes to these properties in the same way that it would ignore changes to any system property. A future improvement might export these changes as for normal config properties under specific circumstances, but that is out of scope for now. Current category injection and override features will apply as they do now. This implies that initial values will never be exported for system properties, including in case of node move. Initial values in existing sources should not be overwritten if other properties of the same node are changed.