Freeciv

Ticket for initial commit of access area module. This is not meant to provide any user visible features yet.

Access areas are path finding determined areas where a specified unit, typically caravan, can travel. This is inspired by some patches ihnatus proposed earlier (in osdn, which is down and I cannot link to specific ticket there)
The main difference to ihnatus' ideas is that he was doing the path finding at real time as part of the requirement resolution, and this access area idea caches access areas at the turn change. I think that the real time path finding solution would be far too CPU heavy for many otherwise good use cases (ruleset implementations)