maintenance: introduce maintenance mode

To be sure you do not harm user connection handled by the gateway, we
are now able to bring the host into maintenance mode. In that mode
the check-gateway script will no longer activate the gateway feature
of batman-adv.

TODO We need to bring down router advertisements to this host as well.
bird6: Rearrange order of execution.

Use notify instead of before.
bird6: service script does not have a status parameter
batman: add firewall chains for traffic on batman interfaces
alfred: Allow connection from batman interfaces
named: Reload instead of restart