(shp_nav_one_sector, mchr[], ship_chr_flags[], Ship-types, Sector-types):

Add M_CANAL flag to indicate which ships can navigate a canal
in a big city.  Remove the hard code values for canal from
shp_nav_one_sector() for which ships can navigate a canal.
Set M_CANAL flag in ship_chr_flags[] to match the hard
coding in shp_nav_one_sector().
Update Ship-types.t with new canal flag.
Update Sector-types.t to indicate that cities have canals.

include/ship.h

@@ -174,6 +174,7 @@ struct mchrstr {
 #define M_CHOPPER	bit(16)	/* can hold choppers */
 #define M_OILER		bit(17)	/* can re-fuel ships */
 #define M_SUPPLY	bit(18)	/* Can supply units/sects/ships */
+#define M_CANAL		bit(19) /* Can navigate a canal (BIG CITY) */
 #define M_ANTIMISSILE   bit(20)	/* Shoot down missile */
 
 #define getship(n, p) \