download: Provide a 'User-Agent' field in HTTP requests.

Fixes <http://bugs.gnu.org/16703>.
Reported by Raimon Grau <raimonster@gmail.com>.

* guix/build/download.scm (http-fetch)[headers]: New variable.
  Pass it as #:headers or #:extra-headers to 'http-get' and
  'http-get*'.