shpsub: Make shp_check_nav() return more useful information

shpsub: Make shp_check_nav() return more useful information

Some callers have to second-guess shp_check_nav() to figure out
whether CN_LANDLOCKED means "too big to fit into the canal" or "can't
go there at all".

Fix that by returning d_navigation.  CN_LANDLOCKED becomes either
NAV_CANAL or NAV_NONE, CN_CONSTRUCTION becomes either NAV_02 or
NAV_60, and CN_NAVIGABLE becomes NAVOK.

The CN_NAVIGABLE, ... codes are now unused.  Drop them.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>