Run systemctl preset-all --global on first boot

Ideally we'd run this on first login per user but that's not supported
yet by systemd so for now let's run it on first boot instead.

Author: DaanDeMeyer

mkosi.extra/usr/lib/systemd/system/preset-global.service

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Unit]
+Before=user.slice
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ConditionFirstBoot=yes
+ExecStart=systemctl preset-all --global