Adds all missing routing attributes and brings the routing
related code to a working state. In the process the API
was broken several times with the justification that nobody
is using this code yet.
The changes include new example code which is also a prototype
for how plain CLI tools could look like to control routes.
So far the destination address for default routes was NULL
which complicated the handling of routes in general. By
assigning a address of length zero they can be compared
to each other.
This allows the cache manager to properly handle default
routes.