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

src/libostree/ostree-sysroot-private.h

@@ -57,9 +57,8 @@ struct OstreeSysroot {
   OstreeDeployment *booted_deployment;
   struct timespec loaded_ts;
 
-  /* Only access through ostree_sysroot_get_repo() */
+  /* Only access through ostree_sysroot_[_get]repo() */
   OstreeRepo *repo;
-  gboolean repo_opened;
 
   OstreeSysrootDebugFlags debug_flags;
 };