gio: Check cancellable iterating local file enumerator

In the typical while (g_file_enumerator_next_file ()) patterns, there is nothing much checking whether the operation was cancelled on the GIO side. Unless the user checks for the case, this means local enumerators always run to completion even if cancelled.

Fix this by checking the cancellable state explicitly for local enumerators, so there are oportunities for bailing out early if the enumerator is going through a very large directory.