As part of allowing different types of oci images to be created,
we split our IMG_cmd into .inc files that can then be specific to
the selected type.

For the umoci backend: We can take the same options as sloci
expects and use umoci to create images.

The resulting OCI image is similar, but by using umoci, we set
the stage to do multi-tag, or multi-layer images in the future.

But for now, we are functionally equivalent to the sloci backend.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>