Support /etc/ostree/remotes.d

For many OS install scenarios, one runs through an installer which may
come with embedded data, and then the OS is configured post-install to
receive updates.

In this model, it'd be nice to avoid the post-install having to rewrite
the /ostree/repo/config file.

Additionally, it feels weird for admins to interact with "/ostree" -
let's make the system feel more like Unix and have our important
configuration in /etc.

https://bugzilla.gnome.org/show_bug.cgi?id=729343

src/libostree/ostree-repo.h

@@ -52,6 +52,8 @@ gboolean      ostree_repo_open   (OstreeRepo     *self,
 void          ostree_repo_set_disable_fsync (OstreeRepo    *self,
                                              gboolean       disable_fsync);
 
+gboolean      ostree_repo_is_system (OstreeRepo   *repo);
+
 gboolean      ostree_repo_create (OstreeRepo     *self,
                                   OstreeRepoMode  mode,
                                   GCancellable   *cancellable,