services: guix: Add 'set-http-proxy' action.

Fixes <https://bugs.gnu.org/25569>.
Reported by Divan Santana <divan@santanas.co.za>.

* gnu/services/base.scm (shepherd-set-http-proxy-action): New procedure.
(guix-shepherd-service): Add 'actions' field.  Change 'start' to a
lambda; check the value of the "http_proxy" environment variable and
add "http_proxy" and "https_proxy" to #:environment-variables as a
function of that.
* gnu/tests/base.scm (run-basic-test)["guix-daemon set-http-proxy
action", "guix-daemon set-http-proxy action, clear"]: New tests.
* doc/guix.texi (Base Services): Document it.