bird4: initial commit

this is mainly the bird6 modules slightly adapted to bird4.
tested, but production usage has to show if everything works
A feature which was requested by @do9xe: We should be able to deactivate default inclusion of bird/bird6.
I would like to have the ability to choose to activate bird and/ or bird6.
Simple reasons, kiel is currently only using bird6 and flensburg is using no bird/bird6/icvpn at all, for political reasons.