install to-filesystem: support separate /var for install

https://github.com/ostreedev/ostree/pull/3166 adds support for an initial `/var` - but we should support configuring this explicitly.

Just like we have `--boot-mount-spec` I'd strawman that we have `--var-mount-spec`.

One interesting question here is whether we should have a mechanism where one can configure this partitioning in the container image too.

Start by reading the existing --boot-mount-spec implementation and the changes in ostree PR #3166 that add an initial /var. Define what --var-mount-spec should configure for filesystem installation, then determine whether container images also need to express this partitioning. Done means explicit separate /var configuration is supported and the image-level question is resolved.

Scritto dal modello di indicizzazione a partire dal testo della issue.