unit_move_handling(): remove igzoc
Create an alternative for the barbarians.
Split unit_move_handling() based on igzoc.
Done as a small step for the sake of ensuring backwards compatibility.
The new function unit_move_igzoc() handles cases where igzoc was set to TRUE. It is kept the exact same rules for when it is allowed to move the unit. (I think some of them should be removed.)
unit_move_handling() keeps - recently enabler controlled - regular moves and the code to understand a move towards a tile as asking what actions the unit can perform against it.