ADD: Public rights of way:
- roads and rivers built on map generation are designated public rights of way;
- players may upgrade these, but they will remain public rights of way;
- all players are able to use public rights of way;
- players may not downgrade public rights of way;
- players may not delete public rights of way without an acceptable diversionary route;
- TODO: allow customisation of the maximum length of the diversionary route in simuconf.tab;
- TODO: implement a mechanism for allowing the public player to add public rights of way later in the game.