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 now, in the upcoming JSON-RPC library and underlying stream library,
there is no reasonable default, so enhance these functions so that they
can require the user to specify a port explicitly.