(att_reacting_units): Use it. Fixes overcharging of inefficient
units. Broken when Empire3 changed land unit mobility use not to
depend on efficiency, except for supply units.
(lnd_sweep): Use it. No functional change.
(speed_factor): New, factored out of lnd_pathcost() and shp_mobcost().
move costs, but failed to make A* use these costs. This broke land
unit path finding. Fix:
(MOB_ROAD, MOB_MOVE, MOB_MARCH): Split MOB_ROAD into MOB_MOVE and
MOB_MARCH. Users changed.
(lnd_mobcost, sector_mcost): Move minimum mobcost logic to
sector_mcost(), where it is visible to A*. Also fixes unit reaction
path cost.
(lnd_path): Fix confusing message: don't claim there's no path when
all we really know is that there's no railway.
