Emit events for docker daemon

This fix tries to cover the issue raised in #22463 by emitting
events for docker daemon so that user could be notified by
scenarios like config reload, etc.

This fix adds the `daemon reload`, and events for docker daemon.

Additional tests have been added to cover the changes in this fix.

This fix fixes #22463.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/commandline/events.md

@@ -35,6 +35,10 @@ Docker networks report the following events:
 
     create, connect, disconnect, destroy
 
+Docker daemon report the following events:
+
+    reload
+
 The `--since` and `--until` parameters can be Unix timestamps, date formatted
 timestamps, or Go duration strings (e.g. `10m`, `1h30m`) computed
 relative to the client machine’s time. If you do not provide the `--since` option,