olsrd: add filtergw plugin

olsrd: add filtergw plugin

The plugin adds a possibility to filter out gateway HNAs. This can be
very handy e.g. for faulty nodes. A gateway can be taken out directly on
a central node and all traffic is then redirected. Also the underlying
smart gateways are adapted. Currently the link multiplier is often used
to control a faulty node, but it is not necessarily the best way to do
so.

Signed-off-by: Nick Hainke <vincent@systemli.org>