netmgr: have a single source of truth for tcpdns callback

We pass interface as an opaque argument to tcpdns listening socket.
If we stop listening on an interface but still have in-flight connections
the opaque 'interface' is not properly reference counted, and we might
hit a dead memory. We put just a single source of truth in a listening
socket and make the child sockets use that instead of copying the
value from listening socket. We clean the callback when we stop listening.

lib/isc/netmgr/netmgr-int.h

@@ -347,7 +347,10 @@ struct isc_nmsocket {
 	int				tid;
 	isc_nmsocket_type		type;
 	isc_nm_t			*mgr;
+	/*% Parent socket for multithreaded listeners */
 	isc_nmsocket_t			*parent;
+	/*% Listener socket this connection was accepted on */
+	isc_nmsocket_t			*listener;
 
 	/*%
 	 * quota is the TCP client, attached when a TCP connection