download: Simplify 'open-connection-for-uri' to support HTTP proxies.

Partly fixes <http://bugs.gnu.org/20402>.
Reported by Joshua Randall <jcrandall@alum.mit.edu>.

* guix/build/download.scm (open-connection-for-uri): Rewrite to be a
  small wrapper around 'open-socket-for-uri'.  This procedure was
  initially introduced in d14ecda9 to work around the lack of NSS modules
  during bootstrap but that has become unnecessary since 06213498, which
  introduced a bootstrap Guile that uses static NSS modules (from commit
  d3b59727.)
  On Guile >= 2.0.10, this allows the 'http_proxy' environment variable
  to be used.