zerotier: add configuration reload trigger

Ensure that the zerotier service is automatically restarted when the uci
configuration is modified.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

net/zerotier/files/etc/init.d/zerotier

@@ -115,3 +115,12 @@ stop_service() {
 	config_foreach stop_instance 'zerotier'
 	rm -f ${CONFIG_PATH}
 }
+
+reload_service() {
+	stop
+	start
+}
+
+service_triggers() {
+	procd_add_reload_trigger 'zerotier'
+}