* guix-download.in (http-fetch, ftp-fetch): Remove.
(fetch-and-store): Replace `uri' parameter with `name', for the output
file name. Redirect the output of `fetch' to the error port.
(guix-download): Call `url-fetch' for all URI schemes except `file'.
Handle PATH equal to #f.
* guix/download.scm: Export `%mirrors'.
* tests/guix-download.sh: Change erroneous URL, because URLs at
example.com are all valid redirections.
* guix/download.scm (%mirrors): New variable. Mirror lists taken from
Nixpkgs.
(url-fetch): New `mirrors' keyword parameter.
[builder]: Pass it.
* guix/build/download.scm (url-fetch): New `mirrors' keyword parameter.
[maybe-expand-mirrors]: New procedure.
[uri]: Use it.
