nixos/auditd: init at 2.7.6 (#27261)

#11864 Support Linux audit subsystem
Add the auditd.service as NixOS module to be able to
generate profiles from /var/log/audit/audit.log
with apparmor-utils.

auditd needs the folder /var/log/audit to be present on start
so this is generated in ExecPreStart.

auditd starts with -s nochange so that effective audit processing
is managed by the audit.service.