modernize cpp echo server

src/emper-echo-server.cpp

 // SPDX-License-Identifier: GPL-3.0-or-later
 // Copyright © 2020 Florian Fischer
-#include <errno.h>
 #include <netinet/in.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 #include <sys/socket.h>
 #include <unistd.h>
 
+#include <cerrno>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
 #include "AsyncLibrary.hpp"
 #include "Common.hpp"
 #include "Fiber.hpp"
 #include "Runtime.hpp"
 #include "config.h"
 
-static int port = 12345;
+static const int DEFAULT_PORT = 12345;
+static int port = DEFAULT_PORT;
 
 [[noreturn]] static void die(const char* msg) {
 	perror(msg);
@@ -35,6 +37,7 @@ void client_func(void* arg) {
 #endif
 		// socket was shutdown
 		if (unlikely(bytes_recv == 0)) {
+			shutdown(client_fd, SHUT_RDWR);
 			return;
 		}
 
@@ -49,7 +52,8 @@ void client_func(void* arg) {
 		ssize_t bytes_send = 0;
 		while (bytes_send < bytes_recv) {
 #ifdef USE_ASYNC_IO
-			ssize_t new_bytes_send = emper::io::send_and_wait(client_fd, buf, bytes_recv - bytes_send, MSG_NOSIGNAL);
+			ssize_t new_bytes_send =
+					emper::io::send_and_wait(client_fd, buf, bytes_recv - bytes_send, MSG_NOSIGNAL);
 #else
 			ssize_t new_bytes_send = send(client_fd, buf, bytes_recv - bytes_send, MSG_NOSIGNAL);
 #endif
@@ -122,7 +126,7 @@ static void welcome_func() {
 	}
 }
 
-int main(int argc, char* argv[]) {
+auto main(int argc, char* argv[]) -> int {
 	if (argc > 2) {
 		const int decimal = 10;
 		port = (int)strtol(argv[1], nullptr, decimal);