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>

include/types.h

@@ -36,6 +36,14 @@
 typedef unsigned char natid;	/* NSC_NATID must match this */
 typedef short coord;
 
+enum d_navigation {
+    NAV_NONE,	/* ships can't navigate */
+    NAVOK,	/* ships can always navigate */
+    NAV_02,	/* requires 2% effic to navigate */
+    NAV_CANAL,	/* requires 2% effic to navigate and M_CANAL capability */
+    NAV_60	/* requires 60% effic to navigate */
+};
+
 struct bp;
 struct emp_qelem;
 struct empobj;