It was possible that controlconf connections could be shutdown twice
when shutting down the server, because they would receive the
signal (ISC_R_SHUTTINGDOWN result) from netmgr and then the shutdown
procedure would be called second time via controls_shutdown().
Split the shutdown procedure from control_recvmessage(), so we can call
it independently from netmgr callbacks and make sure it will be called
only once. Do the similar thing for the listeners.