[freeciv-ai] stay_and_defend() and is_my_turn() cleanup

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

This is a small fix to stay_and_defend() and a larger rewrite of
is_my_turn(), taken from the remnants of the massive ai patch.

stay_and_defend() now doesn't allow ships and helicopters to be city
defenders, and uses a simpler algorithm to check if a unit is a possible
defender or not. This function is still a hack that sucks a lot, though.

is_my_turn() is also an ugly hack, IMHO. Its usefulness will largely be
replaced by AI-only autoattack, so I guess it should go away then. I
cleaned up the code and removed some dubious assumptions.

  - Per

minor1.diff
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