Make ping timeout configurable (fixes #1751)

cmd/syncthing/main.go

@@ -565,6 +565,9 @@ func syncthingMain() {
 		symlinks.Supported = false
 	}
 
+	protocol.PingTimeout = time.Duration(opts.PingTimeoutS) * time.Second
+	protocol.PingIdleTime = time.Duration(opts.PingIdleTimeS) * time.Second
+
 	if opts.MaxSendKbps > 0 {
 		writeRateLimit = ratelimit.NewBucketWithRate(float64(1000*opts.MaxSendKbps), int64(5*1000*opts.MaxSendKbps))
 	}