Style changes and factoring.

	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.