Use substitute servers on the local network.

* guix/scripts/discover.scm: New file.
* Makefile.am (MODULES): Add it.
* nix/nix-daemon/guix-daemon.cc (options): Add "discover" option,
(parse-opt): parse it,
(main): start "guix discover" process when the option is set.
* guix/scripts/substitute.scm (%local-substitute-urls): New variable,
(substitute-urls): add it.
* gnu/services/base.scm (<guix-configuration>): Add "discover?"
field,
(guix-shepherd-service): honor it.
* doc/guix.texi (Invoking guix-daemon): Document "discover" option,
(Base Services): ditto.