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

(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().