Add systemd service file

- When running in an environment without systemd, the changes are completely
  ignored.

- When running under systemd, the patch introduces a systemd native service
  file to take advantages of nice feature provided by systemd.

- One of the feature currently taken advantage of is the ability to restart the
  service automatically if it ever crashes or exits with error.

- Another feature is that when we wish to kill Plinth all the processes under
  the cgroup are killed as well.

data/lib/systemd/system/plinth.service

@@ -0,0 +1,29 @@
+#
+# This file is part of Plinth.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+[Unit]
+Description=Plinth Web Interface
+Documentation=man:plinth(1)
+After=network.target
+
+[Service]
+ExecStart=/usr/bin/plinth --no-daemon
+Restart=on-failure
+ExecReload=/bin/kill -HUP $MAINPID
+
+[Install]
+WantedBy=multi-user.target

setup.py

@@ -133,6 +133,8 @@ setuptools.setup(
                  ['data/etc/apache2/sites-available/plinth.conf',
                   'data/etc/apache2/sites-available/plinth-ssl.conf']),
                 ('/etc/sudoers.d', ['data/etc/sudoers.d/plinth']),
+                ('/lib/systemd/system',
+                 ['data/lib/systemd/system/plinth.service']),
                 ('/usr/share/plinth/actions',
                  glob.glob(os.path.join('actions', '*'))),
                 ('/usr/share/man/man1', ['doc/plinth.1']),