allow tcpdns sockets to self-reference while connected

A TCPDNS socket creates a handle for each complete DNS message.

Previously, when all the handles were disconnected, the socket
would be closed, but the wrapped TCP socket might still have
more to read.

Now, when a connection is established, the TCPDNS socket creates
a reference to itself by attaching itself to sock->self. This
reference isn't cleared until the connection is closed via
EOF, timeout, or server shutdown. This allows the socket to remain
open even when there are no active handles for it.

lib/isc/netmgr/netmgr-int.h

@@ -371,6 +371,8 @@ struct isc_nmsocket {
 	isc_nmsocket_t *parent;
 	/*% Listener socket this connection was accepted on */
 	isc_nmsocket_t *listener;
+	/*% Self, for self-contained unreferenced sockets (tcpdns) */
+	isc_nmsocket_t *self;
 
 	/*%
 	 * quota is the TCP client, attached when a TCP connection