fetchurl: add user agent

It would be nice to be able to track Nix requests. It's not trustworthy,
but can be helpful for stats and routing in HTTP logs.

Since `fetchurl` is used so widely, we should "magically" get a UA on
`fetchzip`, `fetchFromGitHub`, and other related fetchers.

Since `fetchurl` is only used for fixed-output derivations, this should
cause no mass rebuild.

User-Agent example: curl/7.57.0 Nixpkgs/18.03