* 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.in (fetch-and-store): New procedure.
(guix-download): Use it to compute PATH. Call `add-to-store' when
a `file' URI scheme is used.
* Makefile.am (AM_TESTS_ENVIRONMENT): New variable.
* tests/guix-download.sh: Add test.
* guix-download.in (guix-download): Error out when `string->uri'
returns #f. Use `leave' when the scheme is unknown.
* tests/guix-download.sh: Add tests.
* guix-download.in (http-fetch): Leave with an error message when
RESPONSE's code is not 200.
* tests/guix-download.sh: New file.
* Makefile.am (TESTS): Add it.
