switch_to_interface() becomes the common place where
self.interface is set; therefore self.interface is not None
precisely when self.default_server is connected. Previously some
places required it to be connected and some didn't. Also an
interface change now sends the 'updated' notification
consistently - previously some did and some didn't.
Have network_start() call start_interfaces() - this also means
network restarts now do this.
Fix apparent off-by-one in start_interfaces()
86626d8080