[freeciv-ai] AI use airlift v2

["Per I. Mathisen" <per@xxxxxxxxxxx>]

I don't give up just because Greg says so ;)

Here is another version of this patch. Changes:
  - Don't build airport unless we have more than 1/10th as much shield
surplus as the building cost of an Airport (ie 8).
  - More checks to ensure we don't snatch important units.

Granted, with the current ruleset there will rarely be use for this code.
But, once it has been modified slightly to work with gen effects (and I
mean slightly), it should be useful as it enables the AI to use the air
transportation effect to great advantage.

The other diff can be used to test the code. Since I don't expect anyone
to do so, I will commit this patch myself sooner or later, unless someone
(Greg, that's you) speaks up against it.

  - Per

test1.diff
Description: Text document

airline2.diff
Description: Text document