Remove (declare (special ...))s and fix byte-compilation warnings

Which Emacs version was that?

`url-http` does not `defvar` some of these, but `url-vars` does, and it's required via `url-http` -> `url` -> `url-vars`.

To see if a variable is defined as dynamic (or a global: same thing) is a running Emacs session, try `M-x describe-variable`. It plainly won't work on non-globals.