Set device absent when provider is stopped.

lib/units/provider/index.js

@@ -260,6 +260,14 @@ module.exports = function(options) {
             log.info('Device worker "%s" has retired', device.id)
             delete workers[device.id]
             worker = null
+
+            // Tell others the device is gone
+            push.send([
+              wireutil.global
+            , wireutil.envelope(new wire.DeviceAbsentMessage(
+                device.id
+              ))
+            ])
           })
           .catch(procutil.ExitError, function(err) {
             if (!willStop) {