lib/config: Remove "Invalid" attribute (fixes #2471)

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This contains the following behavioral changes:

 - Duplicate folder IDs is now fatal during startup
 - Invalid folder flags in the ClusterConfig is fatal for the connection
   (this will go away soon with the proto changes, as we won't have any
   unknown flags any more then)
 - Empty path is a folder error reported at runtime

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3370

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