lib/deploy: Port config merge logic to new code style

lib/deploy: Port config merge logic to new code style

This is a de-scoping of work I did in preparation for
rpm-ostree [live updates](https://github.com/projectatomic/rpm-ostree/pull/652).
Originally I was going to expose this as a public API.

However, I decided to do things differently, but the cleanup here for new code
style and fd-relative is nice to have anyways.

We rework things to use `OstreeDeployment*`, which the caller is expected to
already have, rather than `GFile*`s pointing to the config directories.

Closes: #741
Approved by: jlebon