[EchoServer] set SO_REUSEPORT on the listen socket

This is needed by emper-io-eval because apparently our startup/termination times are not enough for the OS to allow the rebinding of the same tcp tuple.

Also make all global variables static because they don't have to be exported.

apps/EchoServer.cpp

@@ -21,12 +21,13 @@
 #include <compare>
 #endif
 
-const std::string HOST = "::";
-const std::string PORT = "12345";
+static const std::string HOST = "::";
+static const std::string PORT = "12345";
+static const int BACKLOG = 1024;
 
-unsigned int computations_us = 0;
+static unsigned int computations_us = 0;
 
-std::atomic<bool> quit = false;
+static std::atomic<bool> quit = false;
 
 auto main(int argc, char* argv[]) -> int {
 	std::string host = HOST;
@@ -48,7 +49,7 @@ auto main(int argc, char* argv[]) -> int {
 	std::cout << "Echoserver listening on " << host << ":" << port << std::endl;
 
 	Runtime runtime;
-	auto* listener = emper::io::tcp_listener(host, port, [](int socket) {
+	auto serverFunc = [](int socket) {
 		// NOLINTNEXTLINE(modernize-avoid-c-arrays)
 		char buf[1024];
 		while (!quit.load(std::memory_order_consume)) {
@@ -83,7 +84,10 @@ auto main(int argc, char* argv[]) -> int {
 		}
 
 		emper::io::closeAndForget(socket);
-	});
+	};
+
+	auto* listener =
+			emper::io::tcp_listener(host, port, serverFunc, BACKLOG, {emper::io::SockOpt::ReusePort});
 
 	if (!listener) {
 		exit(EXIT_FAILURE);