[freeciv-ai] Ferries exploring considered harmful

[Per Inge Mathisen <per@xxxxxxxxxxx>]

The AI sends its ferries far, far away exploring distant lands, and
therefore feels a strong need to build new ferries for transporting its
units, and then it sends that ferry, too, exploring in distant lands.
Repeat.

Result: Too many ferries and trouble keeping Republic from revolting.

This is wrong! The AI should be much more conservative in its
explorations. It should explore the area around its cities (to find those
whales and fish), and otherwise let them sit in the cities as the old code
did.

The AI sees all, and at does not need to explore to find other players or
places to place cities. While it should send _some_ ships on exploration,
eventually, it should not sends its vital ferries on such missions.

  - Per