Every remote that sets localpath also implements an availability that
reutrns Unavailable when a local directory is not available.

This makes external remotes, and others that get support for
availability Unavailable to be used by checkAvailable. (Which is only
used by the assistant.)

Had to keep localpath though, since other parts of the assistant use it
to eg, sync with a remote when a removable drive is plugged in.

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