Add native systemd service

Inspired by: https://github.com/pi-hole/pi-hole/pull/2112

A pre-start and a post-stop script are added to reduce doubled setup and cleanup code.

Since systemd services do not natively support dynamic users, test once whether capabilities are supported during install/update, and remove User=pihole otherwise.

Signed-off-by: MichaIng <micha@dietpi.com>
Co-authored-by: DL6ER <dl6er@dl6er.de>

advanced/Templates/pihole-FTL-poststop.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env sh
+
+# Source utils.sh for getFTLPIDFile()
+PI_HOLE_SCRIPT_DIR='/opt/pihole'
+utilsfile="${PI_HOLE_SCRIPT_DIR}/utils.sh"
+# shellcheck disable=SC1090
+. "${utilsfile}"
+
+# Get file paths
+FTL_PID_FILE="$(getFTLPIDFile)"
+
+# Cleanup
+rm -f /run/pihole/FTL.sock /dev/shm/FTL-* "${FTL_PID_FILE}"