fuse3 mount errors are not reliable

[op]

When creating a new mount in fuse3, there are places where errno is used where the function doesn't seem to set an actual errno. This can result in errors like: Success (os error 0) or other random errors.

This part^0 of the Mount::new calls into fuse_session_new^1 and fuse_session_mount^2 which does not seem to directly set an errno.

PS. We tried to upgrade fuser a couple of days ago but some change seem to make things break for us. Haven't had time to figure this out but the errors spotted were very random because of this.