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

(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.