As we expect this project to be mostly run as a system service, provide
out of the box configuration files for systemd.
Notably, we add a .socket file allowing systemd to lazily start kdnotify
when a notify event is received on its socket.
We also hardlink the socket out into the /run/ directory, such that
keepalived does not need to peak inside kdnotify runtime dir.
We also link the lifecycle of kdnotify to keepalived, as once it has
been stopped it is unlikely we'll receive new messages.
