DEV: tcploop: add a new "bind" command to bind to ip/port.

DEV: tcploop: add a new "bind" command to bind to ip/port.

The Listen command automatically relies on it (without passing its
argument), and both Listen and Connect now support working with the
existing socket, so that it's possible to Bind an ip:port on an
existing socket or to create a new one for the purpose of listening
or connecting. It now becomes possible to do:

   tcploop 0 L1234 C8888

to connect from port 1234 to port 8888.