Networking: TCP disconnection callbacks are not retained in a list. This...

Networking:  TCP disconnection callbacks are not retained in a list.  This will support mutiple callbacks per lower-level TCP connection structure.  That is necessary for the cae where a socket is dup'ed and shares the same lower-level connection structure.  NOTE:  There still needs to be a call to tcp_start_monitor() when the socket is dup'ed.