Optionally preserve /home when installing

Optionally preserve (= do not touch) `/home` when it is already existing on a suitable ZFS partitioning scheme.
This would allow for re-installing the OS while keeping user data.

* Disk partitioning and formatting would need to be skipped.
* The existing ZFS datasets would need to be re-used.
* User creation would need to be handled somehow, e.g., by preserving all users with UID >= 1000 from the told to the new system and skipping the user creation dialog.