repo: Introduce ostree_repo_open_at() and ostree_repo_create_at()

repo: Introduce ostree_repo_open_at() and ostree_repo_create_at()

This essentially completes our fd-relative conversion.

While here, I cleaned up the semantics of `ostree_repo_create()` and
`ostree_repo_create_at()` to be more atomic - basically various scripts were
testing for the `objects` subdirectory, so let's formalize that.

Closes: #820
Approved by: jlebon