[freeciv-ai] bigger settlers.c cleanup

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

This includes the previuosly posted removal of is_already_assigned().
Other than that, I did some cleanup of the city map iterations used, and
also imported some changes from massiveai patch.

Changes:
 - Remove is_already_assigned()
 - Remove the settlers.c-specific goto wrapper
 - Use citymap iterators instead of square iterators
 - Style fixes
 - Use handle_unit_activity_request() instead of set_unit_activity()

Please review && commit.

  - Per

settlersc2.diff
Description: Text document