refactor bird6.conf for policy routing

Berlin1 is allowed to send us default routes now
Update list of tincd peers
Implement policy routing for IPv6
Fix some problems with routing

before it happened regulary that locally generated ICMP messages
left eth0 with wrong source address instead of going out via the vpn interface
Improved version of the check_gateway script

in case the vpn is offline, also shutdown radvd and dhcpd so that
clients get a new default gateway when their lease time expires
Let dhcpd also announce NTP servers
Allow to set public IPv6 of the gateway

in addition to the ULA as before. we now have multiple /44 IPv6
networks from the Förderverein
Small fixes (typos)
Increase conntrack size
Ensure new bird version is installed
Merge branch 'master' of github.com:freifunkhamburg/puppet-ff_gw