fetcher: Lazily create tmp directory

The tmp directory is lazily created for each fetcher instance, since
it may require superuser permissions and some instances only need
_ostree_fetcher_request_uri_to_membuf() which keeps everything in
memory buffers.

src/libostree/ostree-fetcher.h

@@ -55,9 +55,7 @@ typedef enum {
 GType   _ostree_fetcher_get_type (void) G_GNUC_CONST;
 
 OstreeFetcher *_ostree_fetcher_new (int                      tmpdir_dfd,
-                                    OstreeFetcherConfigFlags flags,
-                                    GCancellable            *cancellable,
-                                    GError                 **error);
+                                    OstreeFetcherConfigFlags flags);
 
 int  _ostree_fetcher_get_dfd (OstreeFetcher *fetcher);