community/birdwatcher: add secondary initd for ipv6
For each address family a dedicated instance of birdwatcher is required. So add an secondary initd for ipv6 and tune the default config to make it reasonable for bird 2.x+.
For each address family a dedicated instance of birdwatcher is required. So add an secondary initd for ipv6 and tune the default config to make it reasonable for bird 2.x+.