Make land units on reserve mission react within op-area

Make land units on reserve mission react within op-area

Before, they always reacted to their maximum range, and the op-area
was unused.  Change mission() to define the op-area for reserve
missions as well.  Remove the special-case for showing reserve
missions from mission() and show_mission().  New lnd_reaction_range()
factored out of att_reacting_units().  Use it in oprange() to cover
reserve missions.  Pass the mission as separate parameter to oprange()
for now, because miss() doesn't set it in the object until later.