IP address list of all active and broadcasting interfaces.

- The Raspberry Pi can be connected to LAN concurently with
  more interfaces, e.g., with ethernet (eth0) and WiFi (wlan0).
- The function getServerAddress returns sorted list of all IPs,
  so that it needs no input argument anymore.
- If there is no active broadcasting interface available,
  the IP address reads "n.a." clearly indicating this fact.