Don't continue opening a new rndc connection if we are shutting down

Don't continue opening a new rndc connection if we are shutting down

Due to lack of synchronization, whenever named was being requested to
stop using rndc, controlconf.c module could be trying to access an already
released pointer through named_g_server->interfacemgr in a separate
thread.

The race could only be triggered if named was being shutdown and more
rndc connections were ocurring at the same time.

This fix correctly checks if the server is shutting down before opening
a new rndc connection.