[IO] add output file support to echoclient

This is useful for my evaluation. Because with an output file I
just have to check if it exists to see if an echoclient execution was
successful.

apps/EchoClient.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2021 Florian Fischer
+#include <fcntl.h>
 #include <netdb.h>			 //for getaddrinfo
 #include <sys/socket.h>	 // for shutdown, socket, AF_INET
+#include <sys/stat.h>
+#include <unistd.h>
 
 #include <algorithm>	// for find
 #include <atomic>
@@ -158,7 +161,7 @@ static auto existsOption(int argc, char** argv, const std::string& option) -> bo
 static void printUsage(char* name) {
 	std::cerr << "Usage: " << name
 						<< "[-h] [-p <port>] [-c <clients>] [-i <iterations>] [-a <address>] [-s <size>] [-b "
-							 "<server backlog>]"
+							 "<server backlog>] [-f <output-file>]"
 						<< std::endl;
 }
 
@@ -198,6 +201,17 @@ auto main(int argc, char* argv[]) -> int {
 		server_backlog = strtol(server_backlog_s, nullptr, DECIMAL);
 	}
 
+	int out_fd = STDOUT_FILENO;
+
+	char* output_file = getOption(argc, argv, "-f");
+	if (output_file) {
+		out_fd = open(output_file, O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH);
+
+		if (out_fd < 0) {
+			DIE_MSG_ERRNO("opening output file failed");
+		}
+	}
+
 	int err = getaddrinfo(host.c_str(), port.c_str(), nullptr, &server);
 	if (err) {
 		if (err == EAI_SYSTEM) {
@@ -246,8 +260,19 @@ auto main(int argc, char* argv[]) -> int {
 			avg_ns /= 2;
 		}
 
-		std::cout << "clients,iterations,size,time" << std::endl;
-		std::cout << clients << "," << iterations << "," << size << "," << avg_ns << std::endl;
+		std::stringstream sst;
+		sst << "clients,iterations,size,time" << std::endl;
+		sst << clients << "," << iterations << "," << size << "," << avg_ns << std::endl;
+
+		auto output = sst.str();
+		if (emper::io::writeFileAndWait(out_fd, output.c_str(), output.size()) < 0) {
+			DIE_MSG_ERRNO("writing results failed");
+		}
+
+		if (output_file) {
+			emper::io::closeAndForget(out_fd);
+		}
+
 		delete[] client_avgs;
 
 		exit(EXIT_SUCCESS);