fix: shutdown -> close for UDP servers.

- Although shutdown works on Linux to remove a file descriptor
from the internal poll event loop, it isn't a portable approach.
Instead I have replaced the call to shutdown with a call to close.

Author: kcexn

include/net/service/impl/async_udp_service_impl.hpp

@@ -117,8 +117,10 @@ template <typename UDPStreamHandler, std::size_t Size>
 auto async_udp_service<UDPStreamHandler, Size>::stop_() -> void
 {
   using namespace io::socket;
+  using io::socket::close;
+
   auto sockfd = server_sockfd_.exchange(INVALID_SOCKET);
-  shutdown(sockfd, SHUT_RD);
+  close(sockfd);
 }
 
 } // namespace net::service