ostree_repo_checkout_tree_at: New API for checkouts

rpm-ostree currently uses ostree_repo_checkout_tree(), which as a side
effect will use the uncompressed objects cache by default.  This is
rather annoying if you're using rpm-ostree on a server-side
repository, because if you then rsync the repo, you'll be syncing out
the uncompressed objects unless you exclude them.

We added the ability to disable the uncompressed cache in the
repository config to fix this, but it's better to allow application
control over this.  The uncompressed cache will in some future version
become opt in as well.

This new API further:
 - Drops the `GFile` usage in favor of `openat` APIs
 - Improves ergonomics by avoiding callers having to query the source
   `GFileInfo` (and carry around a copy of `OSTREE_GIO_FAST_QUERYINFO`)
 - Has a more extensible options structure

Per the comment, I rather crudely have the `ostree checkout` builtin
call both APIs to ensure some testing coverage.

However, I'd like to in the future have easier-to-set-up testing code
that calls `libtest.sh` to set up dummy data.

doc/ostree-sections.txt

@@ -277,6 +277,7 @@ ostree_repo_write_commit_detached_metadata
 OstreeRepoCheckoutMode
 OstreeRepoCheckoutOverwriteMode
 ostree_repo_checkout_tree
+ostree_repo_checkout_tree_at
 ostree_repo_checkout_gc
 ostree_repo_read_commit
 OstreeRepoListObjectsFlags