Add node autoready oneshot unit

This replicates some of the more important functionality of the defunct
pvc-flush.service unit. On presence of a trigger file (i.e.
/etc/pvc/autoready), it will trigger a "node ready" on boot. It does
nothing on shutdown as this must be handled by other mechanisms, though
a similar autoflush could be added as well.

debian/pvc-daemon-node.postinst

@@ -5,6 +5,7 @@ systemctl daemon-reload
 
 # Enable the service and target
 systemctl enable /lib/systemd/system/pvcnoded.service
+systemctl enable /lib/systemd/system/pvcautoready.service
 systemctl enable /lib/systemd/system/pvc.target
 
 # Inform administrator of the service restart/startup not occurring automatically