install: Support cleanup for alongside installs

[cgwalters]

When we're doing an alongside install we basically leak all storage space from the previous OS. I think the long term cleanest fix is #78

But in the short term what I'm thinking here is we could add a default opinionated systemd unit that does the cleanup. Note that in the fully general case this may have operating system dependent semantics - for example if the previous OS was using e.g. btrfs snapshots or something else. But our default unit I think can do a non-mount traversing loop. (Maybe to be even more safe, we e.g. only traverse files owned by e.g. rpm/dpkg? Doing that would require some more OS integration glue, but tractable)