socket-util: Make TCP open function support no default port.

socket-util: Make TCP open function support no default port.

Until now, tcp_open_active() and tcp_open_passive() have only been used
in situations where there is a reasonable default port, e.g. OFP_TCP_PORT.
But for NetFlow there is no universal default, so enhance these functions
so that they can require the user to specify a port explicitly.

Crossported from the 'db' branch, where this is useful for JSON-RPC, which
also has no widely known port.