Add support for systemd.

listen-for-shutdown.service

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+[Unit]
+Description=Listen for shutdown service at GPIO3
+After=syslog.target network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/python3 /usr/local/bin/listen-for-shutdown.py
+PIDFile=/run/listen-for-shutdown.pid
+
+[Install]
+WantedBy=multi-user.target

script/install.systemd

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+#! /bin/sh
+
+set -e
+
+cd "$(dirname "$0")/.."
+
+echo "=> Installing shutdown listener...\n"
+sudo cp listen-for-shutdown.py /usr/local/bin/
+sudo chmod +x /usr/local/bin/listen-for-shutdown.py
+
+echo "=> Installing shutdown listener systemd service...\n"
+sudo cp listen-for-shutdown.service /etc/systemd/system/
+sudo chmod +x /etc/systemd/system/listen-for-shutdown.service
+
+echo "=> Starting shutdown listener...\n"
+sudo systemctl daemon-reload
+sudo systemctl enable listen-for-shutdown.service
+sudo systemctl start listen-for-shutdown.service
+sudo systemctl status listen-for-shutdown.service
+
+echo "Shutdown listener installed.\n"

script/uninstall.systemd

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+#! /bin/sh
+
+set -e
+
+cd "$(dirname "$0")/.."
+
+echo "=> Stopping shutdown listener...\n"
+sudo systemctl stop listen-for-shutdown.service
+
+echo "=> Removing shutdown listener...\n"
+sudo systemctl disable listen-for-shutdown.service
+sudo systemctl status listen-for-shutdown.service
+sudo rm -f /etc/systemd/system/listen-for-shutdown.service
+sudo rm -f /usr/local/bin/listen-for-shutdown.py 
+
+echo "=> Reloading systemd services...\n"
+sudo systemctl daemon-reload
+echo "Shutdown listener uninstalled.\n"