(lnd_pathcost): New, factored out of lnd_mobcost().

(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().

src/lib/common/move.c

@@ -73,3 +73,9 @@ sector_mcost(struct sctstr *sp, int do_bonus)
 	return MAX(d, LND_MINMOBCOST);
     return MAX(d, 0.01);
 }
+
+double
+speed_factor(double effspd, int tech)
+{
+    return 480.0 / (effspd + techfact(tech, effspd));
+}