Change OstreeFetcher to be dirfd-relative

This is a noticeable cleanup, and fixes another big user of GFile* in
performance/security sensitive codepaths.

I'm specifically making this change because the static deltas code was
leaking temporary files, and cleaning that up nicely would be best if
we were fd relative.

src/libostree/ostree-fetcher.h

@@ -54,8 +54,10 @@ typedef enum {
 
 GType   _ostree_fetcher_get_type (void) G_GNUC_CONST;
 
-OstreeFetcher *_ostree_fetcher_new (GFile                     *tmpdir,
-                                   OstreeFetcherConfigFlags   flags);
+OstreeFetcher *_ostree_fetcher_new (int                     tmpdir_dfd,
+                                    OstreeFetcherConfigFlags   flags);
+
+int  _ostree_fetcher_get_dfd (OstreeFetcher *fetcher);
 
 void _ostree_fetcher_set_proxy (OstreeFetcher *fetcher,
                                 const char    *proxy);
@@ -76,7 +78,7 @@ void _ostree_fetcher_request_uri_with_partial_async (OstreeFetcher         *self
                                                     GAsyncReadyCallback    callback,
                                                     gpointer               user_data);
 
-GFile *_ostree_fetcher_request_uri_with_partial_finish (OstreeFetcher *self,
+char *_ostree_fetcher_request_uri_with_partial_finish (OstreeFetcher *self,
                                                        GAsyncResult  *result,
                                                        GError       **error);