(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.

src/lib/global/nsc.c

@@ -787,6 +787,7 @@ struct symbol sector_navigation[] = { /* for d_nav */
     {NAV_NONE, "land"},
     {NAVOK, "sea"},
     {NAV_02, "harbor"},
+    {NAV_CANAL, "canal"},
     {NAV_60, "bridge"},
     {0, NULL}
 };