units: Add special Desktop Environment user related units

This adds app.slice, session.slice and background.slice.

units/user/app.slice

@@ -0,0 +1,12 @@
+#  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.
+
+[Unit]
+Description=User Application Slice
+Documentation=man:systemd.special(7)

units/user/background.slice

@@ -0,0 +1,12 @@
+#  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.
+
+[Unit]
+Description=User Background Tasks Slice
+Documentation=man:systemd.special(7)

units/user/meson.build

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: LGPL-2.1+
 
 units = [
+        'app.slice',
+        'background.slice',
         'basic.target',
         'bluetooth.target',
         'default.target',
@@ -9,6 +11,7 @@ units = [
         'graphical-session.target',
         'paths.target',
         'printer.target',
+        'session.slice',
         'shutdown.target',
         'smartcard.target',
         'sockets.target',

units/user/session.slice

@@ -0,0 +1,12 @@
+#  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.
+
+[Unit]
+Description=User Core Session Slice
+Documentation=man:systemd.special(7)

units/user/systemd-exit.service

@@ -14,3 +14,7 @@ DefaultDependencies=no
 Requires=shutdown.target
 After=shutdown.target
 SuccessAction=exit-force
+
+[Service]
+# Place into the root slice to not keep another slice unit alive
+Slice=-.slice