With this commit, the allowed networks for ipv4 and ipv6 are
retrieved from the global config file site.conf. An additional
option was added "additional_prefix6" to hold all extra prefixes
beside the main prefix given in "prefix6".

See https://git.c3pb.de/freifunk-pb/ffpb-packages/issues/3

This is NOT a complete package NOR has the current implementation been tested.
It just reflects the state of the work as it exists on my harddrive, hence it
is committed into a feature-branch such that someone else can continue the
work ...