prevent dir listing for more restricted deployment environment

[tsingakbar]

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background

http.FileServer used by golang implemtation will list directory content when the request path is a directory. we deploy novnc for internal usage, but the deploy enviroment is equiped with lots of security restrictions, and dir-listing is not allowed for that. The PR will 403 Forbidden directory listing http request.

[CendioOssman]

I'm afraid some user will want to be able to list files, so this would have to be an optional feature.