Use ISC_R_SHUTTINGDOWN to detect netmgr shutting down

When the dispatch code was refactored in libdns, the netmgr was changed to return ISC_R_SHUTTINGDOWN when the netmgr is shutting down, and the ISC_R_CANCELED is now reserved only for situation where the callback was canceled by the caller.

This change wasn't reflected in the controlconf.c channel which was still looking for ISC_R_CANCELED as the shutdown event.

Closes #3079 (closed)