Multiple /var bugs

[cgwalters]

First, we aren't handling hardlinks, see coreos/rpm-ostree#4820

A simple reproducer is

FROM quay.io/centos-bootc/centos-bootc:stream
RUN mkdir -p /var/lib && echo hello world > /var/lib/foo && ln /var/lib/foo{,2}

The second problem is that our usage of C+ in ostreedev/ostree#3103 isn't in c9s systemd.

One thing I think we could probably do here is synthesize distinct tmpfiles.d snippets instead for the subdirectories of /var instead of having the toplevel.

Alternatively...it is tempting here to try to match the semantics of docker volumes, pretend like the base image has VOLUME /var. Then the idea is that the content from the container is only populated on initial deployment - we don't do any tmpfiles.d stuff.

The more I think about the more I like that.

[cgwalters]

xref systemd/systemd@30e59a0

[cgwalters]

ostreedev/ostree#3166