[freeciv-ai] (PR#4035) Separate stack_cost and move into aitools

["Gregory Berkolaiko" <Gregory.Berkolaiko@xxxxxxxxxxxx>]

This patch separates a function which calculate the cost of our target
including the units which will die in a successful attack and places it
into aitools.c.  Until now it has been a part of the aiair.c code.

Idea space:  is_stack_vulnerable should be made part of the common code
(after all it's the game-rules) and kill_unit should make use of it.

I feel it's a separate patch though...

G.

stack_cost.diff
Description: stack_cost.diff