Support: run JuiceFS in background in Windows

[diluga]

JuiceFS does not run in the background of Windows as of now。

https://github.com/juicedata/juicefs/blob/main/cmd/mount_windows.go#L48

func makeDaemon(c *cli.Context, name, mp string, m meta.Meta) error {
	logger.Warnf("Cannot run in background in Windows.")
	return nil
}

It is recommended to use the following tools to achieve cross-platform compatibility.

https://github.com/kardianos/service

[xiaogaozi]

As a workaround, you could use NSSM run JuiceFS as a Windows service. There's a user guide (written in Chinese) show how to do this (thanks @diluga).