So it can be shared with the deployment path.  Prep for dropping
`ex-integrity.composefs`.

Handle a NULL pointer.

I want to try to get away from the "repository global" configuration
in the repo config.

A major problem is that there's not an obvious way to configure
it as part of an ostree commit/container build - it needs
to be managed "out of band".

With this change, we parse the `usr/lib/ostree/prepare-root.conf`
in the deployment root, and if composefs is enabled there,
then we honor it.

We do still honor `ex-integrity.composefs` but that I think
we can schedule to remove.