Use 'r+' with truncation when saving existing files on Windows

[seishun]

Opening a file with 'w' flag removes its alternate data streams. To
prevent this, extend the logic used for hidden files to all existing
files.

Fixes: #6363

[WSLUser]

@bpasero Can this be reviewed so we can resolve #49021?

[andrewmackrodt]

I think this would be better as a configuration item (default off) for all platforms. It's becoming quite common for editors to support the ability to disable safe writes (i.e. enable save in place); I'd imagine this would therefore be of benefit to Mac OS and Linux files using ACLs?

[drujd]

This also fixes #49021

[drujd]

@bpasero Is there a chance this gets merged? This would be a blessing for all WSL users.

[bpasero]

Thanks for the PR, I think we should try to go with it as it seems like the only solution. I fear a little bit the consequences this could have compared to how we save today, but our insiders hopefully reveal if there are any. If there are issues we might have to put this behind a setting.