4.0.2 made land unit mobility costs differ significantly from normal

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.

src/lib/update/finish.c

@@ -220,7 +220,7 @@ assemble_dist_paths(struct distinfo *distptrs)
 	    p = ReversePath(path);
 	    /* And walk the path back to the dist center to get the export
 	       cost */
-	    infptr->excost = pathcost(sp, p, MOB_ROAD);
+	    infptr->excost = pathcost(sp, p, MOB_MOVE);
 #ifdef SAVE_FINISH_PATHS
 	    memcpy(infptr->path, p, len);
 #else