build, etc: Add systemd units and ufw rules for relay and discovery (fixes #5115) (#5350)

cmd/strelaysrv/scripts/preinst

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+addgroup --system syncthing
+adduser --system --home /var/lib/syncthing-relaysrv --ingroup syncthing syncthing-relaysrv