cmd/syncthing, lib/config, lib/osutil: Lower process priority (fixes #4628)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4675

cmd/syncthing/main.go

@@ -915,6 +915,12 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 
 	cleanConfigDirectory()
 
+	if cfg.Options().SetLowPriority {
+		if err := osutil.SetLowPriority(); err != nil {
+			l.Warnln("Failed to lower process priority:", err)
+		}
+	}
+
 	code := <-stop
 
 	mainService.Stop()