Disable the cancellation callback during shutdown.

Several backends require a shutdown command to be send before closing
the connection. If such a command gets cancelled, that might result in
an unclean shutdown. Backends where this is problematic can always
ignore cancellation requests internally, but it's less error-prone (and
much easier) to simply disable the cancellation callback for all
backends, before closing the connection.

src/device.c

@@ -315,6 +315,10 @@ dc_device_close (dc_device_t *device)
 	if (device->vtable->close == NULL)
 		return DC_STATUS_UNSUPPORTED;
 
+	// Disable the cancellation callback.
+	device->cancel_callback = NULL;
+	device->cancel_userdata = NULL;
+
 	return device->vtable->close (device);
 }