download: Work around Guile small-receive-buffer bug.

download: Work around Guile small-receive-buffer bug.

Previously, code using directly (guix build download) was still affected
by <http://bugs.gnu.org/15368>.  This includes source derivations, the
'guix download' command, and (guix gnu-maintenance).

'guix substitute' was unaffected since it used (guix http-client), which
already had the fix.

* guix/http-client.scm (open-socket-for-uri): Remove.
  (http-fetch): Remove #:buffered? argument to 'open-socket-for-uri';
  use 'setvbuf' instead.
* guix/scripts/substitute.scm (fetch): Likewise.
* guix/build/download.scm (open-socket-for-uri): New procedure, taken
  from guix/http-client.scm, but without the #:buffered? parameter.