Added alternate local import procedure using podman unshare and mount commands

[Madeeks]

This PR proposes an alternative procedure to import images from local Podman repositories.

Instead of exporting a Podman container filesystem to tar and re-extracting said archive, the podman unshare and podman mount commands are used to mount the container filesystem in a host-accessible location which can in turn be used as target for image configuration and squashing.

The new procedure

• is much faster: it skips tar creation and extraction, leaving squashfs creation as the only time-consuming part of the import
• is conceptually more consistent with the process of importing remote images, where layers are mounted with overlayfs to generate the rootfs to be squashed.

The new procedure is accessed through the -x / --extract-mode option to enroot import, e.g. enroot import -x mount podman://ubuntu:latest.
The previous procedure based on tar export and extraction remains as default.

[flx42]

Is your goal to generate a squashfs and distribute it? Or is your next step enroot create with the squashfs imported from podman?
Because if your use case is the latter, we should consider adding this feature for the enroot load path instead I think.