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

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.