build: Default to "https://mirror.hydra.gnu.org/" for substitutes.

* config-daemon.ac: Check for (gnutls) and define 'GUIX_SUBSTITUTE_URLS'.
* nix/nix-daemon/guix-daemon.cc (main): Use GUIX_SUBSTITUTE_URLS.
* guix/store.scm (%default-substitute-urls): Use 'https' when (gnutls)
is available.
* doc/guix.texi (Binary Installation): Mention mirrors
(Invoking guix-daemon): Mention mirror.hydra.gnu.org.
(Substitutes): Mention mirrors.
(Invoking guix archive): Show https URLs.