Add OstreeRepo option for an out-of-band cache dir

This allows you to have a writable cache dir even for a system-owned
repository.

Closes: #250
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -63,6 +63,13 @@ _OSTREE_PUBLIC
 void          ostree_repo_set_disable_fsync (OstreeRepo    *self,
                                              gboolean       disable_fsync);
 
+_OSTREE_PUBLIC
+gboolean      ostree_repo_set_cache_dir (OstreeRepo    *self,
+                                         int            dfd,
+                                         const char    *path,
+                                         GCancellable   *cancellable,
+                                         GError        **error);
+
 _OSTREE_PUBLIC
 gboolean      ostree_repo_get_disable_fsync (OstreeRepo    *self);