A label design is threatening to erupt into a legal war between two of Japan's biggest brewers Kirin Brewery and Sapporo Breweries Ltd. Kirin is threatening to sue its rival because it believes the label design on Sapporo Fine Lager beer is a copy of its Kirin Lager Beer.   Kirin has already sent Sapporo a written notice demanding it replace the red and white design after it received over 100 complaints from consumers saying they had confused the two products, Kirin said. It claims the label infringes laws on unfair competition.