1. No default socket name is provided. If Why3 should connect to
   an external server, Prove_client.set_socket_name must be called
   first.

2. If Prove_client.set_socket_name was never called, Why3 assumes
   to be in the stand-alone mode, and will start the server in the
   standard tmp directory (on Unix, $TMPDIR or /tmp, on Win, $TEMP or .).
   The socket name is set to be "why3server.<PID_of_Why3>.sock",
   to avoid clashes with other Why3 processes (fixes the Coq tactic).

3. Global reference Prove_client.standalone is removed.

4. Call_provers does not reexport Prove_client.set_socket_name.

5. Prove_client.connect does nothing if the socket reference is set.