netmgr fixes:

- use UV_{TC,UD}P_IPV6ONLY for IPv6 sockets, keeping the pre-netmgr
   behaviour.
 - add a new listening_error bool flag which is set if the child
   listener fails to start listening. This fixes a bug where named would
   hang if, e.g.,  we failed to bind to a TCP socket.

lib/isc/netmgr/netmgr-int.h

@@ -390,6 +390,7 @@ struct isc_nmsocket {
 	 */
 	atomic_bool	      	closed;
 	atomic_bool	      	listening;
+	atomic_bool		listen_error;
 	isc_refcount_t	      	references;
 
 	/*%