services: guix: Make /etc/guix/acl really declarative by default.

Fixes <https://bugs.gnu.org/39819>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

* gnu/services/base.scm (substitute-key-authorization): Symlink
DEFAULT-ACL to /etc/guix/acl unconditionally.  Add code to optionally
back up /etc/guix/acl if it was possibly modified by hand.
* doc/guix.texi (Base Services): Clarify the effect of setting
'authorize-keys?' to true.  Mention the backup.  Give an example showing
how to authorize substitutes from another server.

gnu/services/virtualization.scm

@@ -875,7 +875,16 @@ that will be listening to receive secret keys on port 1004, TCP."
                          (permit-root-login #t)
                          (allow-empty-passwords? #t)
                          (password-authentication? #t)))
-               %base-services/hurd))))
+
+               ;; By default, the secret service introduces a pre-initialized
+               ;; /etc/guix/acl file in the childhurd.  Thus, clear
+               ;; 'authorize-key?' so that it's not overridden at activation
+               ;; time.
+               (modify-services %base-services/hurd
+                 (guix-service-type config =>
+                                    (guix-configuration
+                                     (inherit config)
+                                     (authorize-key? #f))))))))
 
 (define-record-type* <hurd-vm-configuration>
   hurd-vm-configuration make-hurd-vm-configuration