- storage.conf: comment out options that are not supported by btrfs.
This simplifies switching the driver as it avoids the whack-a-mole
of commenting out "unsupported" options.

- Consolidate libcontainers-{common,image,storage} into one package,
libcontainers-common. That's the way upstream intended all libraries from
github.com/containers to be packaged. It facilitates updating and maintaining
the package, as all configs and manpages come from a central source.
Note that the `storage` binary that previously has been provided by the
libcontainers-storage package is not provided anymore as, despite the claims
in the manpages, it is not intended for production use.

- Make libcontainers-common arch independent.
- Add LICENSE.