Don't include raiding units in Units::isActive; make getPosition return invalid coords

[Bumber64]

Raiding units are unflagged as inactive shortly after they leave the map, as they are removed from units.active. Scan this vector to rule out raiding units. The move_state and can_swap flags are also unset, which can be used to optimize the check.

Units::getPosition now returns invalid coords for inactive units. This impacts Units::isUnitInBox, and Units::getUnitsInBox.

It also impacts Items::getPosition, but an item in items.other.IN_PLAY has its holder is guaranteed to be active (artifact theft bugs notwithstanding).

Only iterate active units in spectate instead of all. (Mainly for efficiency. Units::isActive change would've caught any potential issue with raiding units.)