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.
agent: Processing agent side originted messages with route change

VnswInterfaceListener is now able to react to events requiring it
to alter route, originating from the agent side.
agent: Processing agent side originted messages with route change

VnswInterfaceListener is now able to react to events requiring it
to alter route, originating from the agent side.

Post-review changes.