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.

include/sect.h

@@ -179,8 +179,9 @@ extern struct dchrstr bigcity_dchr;
 #define FORTEFF 5		/* forts must be 5% efficient to fire. */
 
 #define MOB_NONE    0
-#define MOB_ROAD    1
-#define MOB_RAIL    2
+#define MOB_MOVE    1
+#define MOB_MARCH   2
+#define MOB_RAIL    3
 
 #define INT_ROAD    0
 #define INT_RAIL    1