download: Use the 'SERVER NAME' TLS extension when possible.

Fixes <http://bugs.gnu.org/18526>.
Reported by Mark H. Weaver.

* guix/build/download.scm (tls-wrap): Add 'server' parameter.  Call
  'set-session-server-name!' when (gnutls) defines it.
  (open-connection-for-uri): Adjust 'tls-wrap' call accordingly.