Units should not pursue enemy units if they are not visible

[Andrew]

Units, which pursue the enemy, should only go as far as they last saw the unit. If when they get there, they don't see it, they return to where they were originally. If they see the unit, or other enemy unit, they pursue until they kill it, or lose it. Again, in both cases, they should return to where they started off, not remain where they lost/killed the enemy unit.

[Kieran P]

For Alpha 2, until the above conditions are implemented, a simple change to stop follow orders if the units goes into FoW or SoD would suffice for now. A unit should not be able to follow another unit all around the world, esp when the other unit is much faster.

[Kieran P]

Pushing back to Alpha 3 due to time constraints and complexity.

[philip]

(In [9657]) Attempt minor improvements to stances code (see #865). Don't chase units that are no longer visible, beyond where they were last seen (fixes #595).