Add support for additional, read-only data stores

Some users may have direct access to additional data stores in
gdatastore format. Allow the user to "register" them by placing links
inside the (new) "extra-stores" directory and use them as additional
sources.

Modification is only supported for the primary store. The additional
ones are used for reading only. In particular, trying to remove an
entry that's contained in an extra store will yield an error (no
shadowing).

The extra stores will need to use exactly the same on-disk format as
the primary one. No automatic migration is done.

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