import: add new minimal tool "systemd-import" for pulling down foreign containers...

import: add new minimal tool "systemd-import" for pulling down foreign containers and install them locally

This adds a simply but powerful tool for downloading container images
from the most popular container solution used today. Use it like
this:

       # systemd-import pull-dck mattdm/fedora
       # systemd-nspawn -M fedora

This will donwload the layers for "mattdm/fedora", and make them
available locally as /var/lib/container/fedora.

The tool is pretty complete, as long as it's only about pulling down
images, or updating them. Pushing or searching is not supported yet.