activation: Shared system home directories are now 555 and root-owned.

activation: Shared system home directories are now 555 and root-owned.

Fixes <https://bugs.gnu.org/34788>.
Reported by Jack Hill <jackhill@jackhill.us>.

Regression introduced by the combination of
8bb76f3d44c1f5ffec8011819494db306a51d801 and
0ae735bcc8ff7fdc89d67b492bdee9091ee19e86: /var/empty would be 700 and
owned by one of the system accounts (thus inaccessible to others), and
/var/run/dbus would be 700 as well, thereby preventing D-Bus clients
from connecting to the daemon.

* gnu/build/activation.scm (duplicates): New procedure.
(activate-users+groups)[system-accounts]: New variable.
Use it.  Make shared system account home directories #o555 and
root-owned.
* gnu/services/dbus.scm (dbus-activation): Make /var/run/dbus #o755.
* gnu/tests/base.scm (run-basic-test): Test the ownership and
permissions of /var/empty.