(fuel, load, shp_check_nav, retreat_ship1, shp_nav_one_sector)

(shp_check_nav, sail_nav_fleet, bigcity_dchr[], sector_navigation[])
(d_navigation): Add a NEW d_navigation enum NAV_CANAL to
indicate that a sector has canal capability. Add canal determination
logic to shp_check_nav(). Update sector_navigation[] with new
canal navigation enum.  Use shp_check_nav() to determine the
sector can be navigated.

include/sect.h

@@ -93,6 +93,7 @@ typedef enum {
     NAV_NONE,	/* ships can't navigate */
     NAVOK,	/* ships can always navigate */
     NAV_02,	/* requires 2% effic to navigate */
+    NAV_CANAL,	/* requires 2% effic to navigate and M_CANAL capability */
     NAV_60	/* requires 60% effic to navigate */
 } d_navigation;