agent: Interface/Address/Route change in system observers

This commit includes parsing of PF_ROUTE communication for the
purpose of reacting to in-system changes in Interface, address
and route configuration.
This an error. We are interested in UNICAST/HOST routes only. This can be verified by looking at the entry condition of HandleNetlinkRouteMsg.
Additionally it would be good to do the DEBUG logging as in the HandleNetlinkRouteMsg for messages that will not be processed, due to them failing to fit to requirements.