Monitor a PFROUTE socket on *BSD and macOS.

Catta is not handling network changes correctly on *BSD and macOS. In
particular, after the initial startup, interfaces that go up and down
do not cause a callback to be generated, so MeshLink is not notified of
the changes.

To ensure MeshLink responds rapidly to network changes on these
platforms, we open a PFROUTE socket and monitor it ourself. We only
check the message type, and don't track exactly what addresses get added
or removed.