Windows: add option to startup juicefs as system service. fix #347, link #1546

[chenjie4255]

fix #347, link #1546

When the --as-svc mount option is specified, the juicefs mount command updates the Windows registry to prepare the service definition needed by the Winfsp Launcher. After updating the registry, the juicefs mount process invokes net use to trigger the mount behavior.

https://github.com/winfsp/winfsp/wiki/WinFsp-Service-Architecture

Known limitations:

• Only one volume can be mounted at a time when using --as-svc. Mounting more than one volume may cause unexpected behavior depending on the implementation of winfsp.
• We essentially write the same command line to the winfsp service registry value, except for the --as-svc option, including the mount point. Therefore, when users use the “Map a network drive” feature in File Explorer, the mount process will ignore the mount point.

Test

• Run command with "--as-svr" option to mount volume 1 at x:.
• Disconnect volume 1 mounting from File Explorer.
• Run command with "--as-svr" option to mount volume 2 at y:

[chenjie4255]

changed to the "-d" and the "--background" options, and re-ran the previous tests.

To simplify the modification, I did not make changes to the existing stage < 3 code. (Stage was forcibly set to 3 on Windows system at earlier code)