bug#25569: Changing the HTTP/FTP proxy used by the daemon is inconvenient

[Ludovic Courtès]

Changing the HTTP{,S} and FTP proxies used by guix-daemon is
inconvenient, especially when doing a GuixSD install, as reported by
Divan Santana <address@hidden> at:

  https://lists.gnu.org/archive/html/help-guix/2017-01/msg00047.html

We should add a service-specific action to do that, like:

  herd set-http-proxy guix-daemon http://example.org

or, alternately, let guix-daemon (‘guix substitute’ and ‘guix
perform-download’) honor client-provided proxy settings passed via
‘set-build-options’ or similar.

Ludo’.