Style changes and factoring.

* src/named-ref.h: Add comments.
	* src/parse-gram.y: Readability and style changes.
	* src/reader.c: Factoring: assign_named_ref function.
	* src/scan-code.l: Factoring and style changes. Rename
	parse_named_ref to parse_ref. Use "c-ctype.h" from gnulib.
	Use "unsigned" type for variant index. Improve readablity.
	* src/scan-gram.l: Change error messages and add comments.
	* src/symlist.h: symbol_list_null: New function decl.
	* src/symlist.c: symbol_list_null: Implement here.
	* tests/named-refs.at: Adjust for new error messages.

src/symlist.h

@@ -114,6 +114,9 @@ symbol_list *symbol_list_n_get (symbol_list *l, int n);
    symbol N in rule RULE.  */
 uniqstr symbol_list_n_type_name_get (symbol_list *l, location loc, int n);
 
+/* Check whether the node is a border element of a rule. */
+bool symbol_list_null (symbol_list *node);
+
 /** Set the \c \%destructor for \c node as \c code at \c loc.  */
 void symbol_list_destructor_set (symbol_list *node, char const *code,
                                  location loc);