fix(services/sftp): change default root config to remote server setting

[silver-ymz]

Previously, sftp root config only supports absolute path. This patch is mentioned in #2192.

[Xuanwo]

It is intended that the root path must be an absolute path. User must specify the correct root path for sftp services. But we can use the server's path as default.

If users doesn't set root, we will use the user's home directory as root.

We can mention this in root config. Please also add a new test job for this case (root is empty).

[silver-ymz]

Update:

• change default root to remote server setting.
• add a new test job for it

The default root path in sftp can be set by both ChrootDirectory and sftp-server argument. And ChrootDirectory will change the apparent root directory, which means the program can't access files and commands outside that environmental directory tree. If ChrootDirectory is set to /home/foo, the previous implementation will create /home/foo/home/foo. I think it's an inappropriate behavior. So, in current implementation, it will do nothing if the user don't set root config.

About the github action test, because atmoz/sftp limits users to write the root directory, I have to modify /etc/ssh/sshd_config to change its initial entry directory.

Could you make a review about it?

[Xuanwo]

Mostly LGTM!

[silver-ymz]

All review suggestions have accepted. Could you make the final review?

[Xuanwo]

Thanks！