Make `maxRange` work on weaponless units

`Spring.SetUnitMaxRange(unitID, number)` sets a unit's engagement range. It controls how close a unit will walk up to enemies. You can make sure a unit with multiple weapons walks to an optimal range without getting confused by sidearms with different ranges.

It would be good if this worked regardless of whether the unit actually has any weapons. This way utility units (radar truck etc) on a fight command would be able to be configured to stop alongside other combat units and not drive into the enemy.

Start by tracing the C++ implementation of Spring.SetUnitMaxRange(unitID, number) and how engagement range currently depends on a unit's weapons. Check the fight-command behavior for weaponless utility units, then verify that maxRange makes them stop at the configured range without changing the existing multi-weapon behavior.

Written by the indexing model from the issue text.