(fuel, load, shp_check_nav, retreat_ship1, shp_nav_one_sector)

(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.