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.
One more thing. There are two possible ways to set unicast route, which do differ in structure of message. One way is to provide destination with netmask 255.255.255.255 and gatway and no RTF_HOST flag, the other is to give destination without netmask with gateway and RTF_HOST set. Bot should be served the same way.