(success): Remove var.
(url-http-handle-authentication): Return the value that `success'
should take instead of setting `success' directly. Don't set `status'
since it's not used.
(url-http-parse-headers): Avoid unneeded setq.
Move the `setq success'.
(url-http): Use pcase.
(url-http-file-exists-p): Simplify.