* Add ISDN support.

* Let the user select the device name in the Properties dialog -
  some people are using syncppp and thus require ippp0/sppp0.

* Add detection for when the selected device is not responding. Stop trying to
  poll the device and give a warning message.

* Add testing of modem lock file to check if the PID listed in that file is
  valid, sometimes the lock file is not deleted when the connection breaks.