server.Shutdown() should print errors

[leventov]

https://github.com/grandcat/zeroconf/blob/c2d1b4121200e6bf8490af806aa6f3dc6d37446b/server.go#L195-L198

Should be

if err := s.shutdown(); err != nil {
  log.Printf("Error shutting down the server: %v", err)
}

shutdown() itself should propagate or print errors from closing ipv4conn and ipv6conn.