Use ISC_R_SHUTTINGDOWN to detect netmgr shutting down

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.