The idea of a common handle is long revised and only misleading,
nl_handle really represents a socket with some additional
action handlers assigned to it.
Alias for nl_handle is kept for backwards compatibility.
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.