Fix *_ctx_new and *_ctx_free implementations

They both will only alloc and free the internal *_ctx structs. Get rid
of the void * argument for new and only pass arg to *_free instead of
the whole lc_*_ctx struct.

auth.h

@@ -25,7 +25,7 @@ struct lc_auth_impl {
 	int	 (*auth)(const uint8_t *, size_t, uint8_t *, size_t *,
 		    const uint8_t *, size_t);
 
-	void	*(*ctx_new)(const void *);
+	void	*(*ctx_new)(void);
 	void	 (*ctx_free)(void *);
 };