This creates a role for RFC3549 Netlink monitoring.
If the OS supports it (currently, linux) then each pt creates a wsi
with the netlink role and dumps the current routing table at pt init.
It then maintains a cache of the routing table in each pt.
Upon routing table changes an SMD message is issued as an event, and
Captive Portal Detection is triggered.
All of the pt's current connections are reassessed for routability under
the changed routing table, those that no longer have a valid route or
gateway are closed.
