modify reference counting within netmgr

- isc__nmhandle_get() now attaches to the sock in the nmhandle object.
  the caller is responsible for dereferencing the original socket
  pointer when necessary.
- tcpdns listener sockets attach sock->outer to the outer tcp listener
  socket. tcpdns connected sockets attach sock->outerhandle to the handle
  for the tcp connected socket.
- only listener sockets need to be attached/detached directly. connected
  sockets should only be accessed and reference-counted via their
  associated handles.

lib/isc/netmgr/netmgr-int.h

@@ -405,6 +405,7 @@ struct isc_nmsocket {
 	int nchildren;
 	isc_nmiface_t *iface;
 	isc_nmhandle_t *tcphandle;
+	isc_nmhandle_t *outerhandle;
 
 	/*% Extra data allocated at the end of each isc_nmhandle_t */
 	size_t extrahandlesize;
@@ -589,6 +590,9 @@ isc__nmhandle_get(isc_nmsocket_t *sock, isc_sockaddr_t *peer,
  *
  * If 'local' is not NULL, set the handle's local address to 'local',
  * otherwise set it to 'sock->iface->addr'.
+ *
+ * 'sock' will be attached to 'handle->sock'. The caller may need
+ * to detach the socket afterward.
  */
 
 isc__nm_uvreq_t *