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: 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.

Post review fixes.
Merge pull request #12 from Semihalf/dev-vnswif-listener-freebsd

agent: Interface/Address/Route change in system observers