dict: lookups take a trie; a prefix search is its own function; no self-key rule

dictlookup took a Lang for two reasons that were not its business: to
know whether to walk below the key (the emoji dictionary) and to drop a
candidate equal to the key except there.  No dictionary lists its key
among its candidates — the rule was left from a first version that put
the reading first itself — so it goes, and the walk is dictprefix(),
called by the emoji search alone.

fn.h

@@ -21,7 +21,8 @@ int	trielookup(Trie*, Str*, char**, int*);
 Lang*	getlang(int);
 void	langinit(char*);
 
-int	dictlookup(Lang*, Str*, Str*, int);
+int	dictlookup(Trie*, Str*, Str*, int);
+int	dictprefix(Trie*, Str*, Str*, int);
 
 void	drawthread(void*);
 void	popuparea(Area*, int, Area*, int, int, Area*);