cmd/xdpderper,derp/xdp: implement mode that drops STUN packets (#12527)

This is useful during maintenance as a method for shedding home client
load.

Updates tailscale/corp#20689

Signed-off-by: Jordan Whited <jordan@tailscale.com>

derp/xdp/xdp_default.go

@@ -26,3 +26,11 @@ func (s *STUNServer) Close() error {
 func (s *STUNServer) Describe(descCh chan<- *prometheus.Desc) {}
 
 func (s *STUNServer) Collect(metricCh chan<- prometheus.Metric) {}
+
+func (s *STUNServer) SetDropSTUN(v bool) error {
+	return errors.New("unimplemented on this GOOS")
+}
+
+func (s *STUNServer) GetDropSTUN() bool {
+	return true
+}