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/libostree-devel.sym

@@ -21,6 +21,8 @@
 LIBOSTREE_2017.10 {
   ostree_gpg_error_quark;
   ostree_repo_set_alias_ref_immediate;
+  ostree_repo_open_at;
+  ostree_repo_create_at;
 };
 
 /* Stub section for the stable release *after* this development one; don't