TOKYO (Reuters) - Japan should be prepared to shoot down ballistic missiles bound for the United States although this would relax its self-imposed ban on collective self-defense, or aiding an ally under attack, government advisers said on Friday. North Korea, which launched a Taepodong ballistic missile in 1998 that overflew Japan, is believed to have missiles capable of reaching the United States, Japan's closest security ally. Washington has made clear it would welcome an end to the collective self-defense ban. "There was overall consensus that it is absurd to have a legal system where Japan can't do anything," said Shinichi...