Hook up systemd-tmpfiles as user units

An explicit --user switch is necessary because for the user@0.service instance
systemd-tmpfiles is running as root, and we need to distinguish that from
systemd-tmpfiles running in systemd-tmpfiles*.service.

Fixes #2208.

v2:
- restore "systemd-" prefix
- add systemd-tmpfiles-clean.{service,timer}, systemd-setup.service to
  systemd-tmpfiles(8)

presets/meson.build

@@ -17,3 +17,6 @@
 
 install_data('90-systemd.preset',
              install_dir : systempresetdir)
+
+install_data('user/90-systemd.preset',
+             install_dir : userpresetdir)

presets/user/90-systemd.preset

@@ -0,0 +1,14 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+# These ones should be enabled by default, even if distributions
+# generally follow a default-off policy.
+
+enable systemd-tmpfiles-setup.service
+enable systemd-tmpfiles-clean.timer