lib/cleanup: Port some of the cleanup code to fd-relative and new style

There aren't many users of `g_file_enumerator_iterate()` left - those
remaining are usually good candidates for porting.  There's some more
porting to do in this file; a mix of trivial and harder.  This
one is a good candidate for an individual commit.

Closes: #803
Approved by: jlebon

src/libostree/ostree-sysroot-private.h

@@ -87,7 +87,8 @@ _ostree_sysroot_parse_deploy_path_name (const char *name,
                                         GError    **error);
 
 gboolean
-_ostree_sysroot_list_deployment_dirs_for_os (GFile               *osdir,
+_ostree_sysroot_list_deployment_dirs_for_os (int                  deploydir_dfd,
+                                             const char          *osname,
                                              GPtrArray           *inout_deployments,
                                              GCancellable        *cancellable,
                                              GError             **error);