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.
This interface was internal so far which required all code defining
caches to be compiled with the sources available.
In order to simplify the interface, the co_msg_parser prototype was
changed to take the struct nl_parser_param directly instead of a
void *. It used to be void * because the co_msg_parser was directly
passed as the NL_CB_VALID callback function.
