install: Support reinstallation (factory reset)

https://github.com/containers/bootc/pull/137 is tracking part of this, a lot of related discussion in https://github.com/coreos/fedora-coreos-tracker/issues/399

Basically we should support something like `bootc install factory-reset` that would configurably wipe local system state.

On the implementation side...I think the best way to do this would be something like:

- Create new deployment with fresh `etc` targeting a new stateroot
- Boot into it
- optional: clean up old deployment + stateroot

The beauty of this approach is that the "factory reset" would actually also be fully transactional, i.e. you could roll back into the previous state. It'd be easy to copy over any desired state (SSH host keys, cached container images e.g.) from the previous boot before deleting it.

## Active work

- https://github.com/bootc-dev/bootc/pull/1389

## Goals:

- Make stateroots more visible, e.g. at least status should show them if different
- Need to support explicit pruning of deployments e.g. `bootc deployment rm` (like `ostree admin undeploy`) so that I can get rid of old stateroots

## Ideas:

- Add `bootc mount /run/tmp-next-root` so that I can `cp -a --reflink=always /var/home /run/tmp-next/root/var/home`