PeriodicGPS: refactor to validate preferences on service start

[iddq]

minimum update distance should be zero if no value specified.

[ge0rg]

Thanks for the report. I understand that this is slightly confusing, however it is a safety measure to prevent accidentally setting up a 0/0 configuration. I need to rework the PeriodicGPS preferences to be more generous in what it accepts, and to change the code on startup to check the values and to abort with a respective message if they are not proper.