threadsafe accept on windows, linux

* Windows has WSAAccept() which will create sockets inheriting flags from
  the server socket

* Linux has accept4() which has a flags argument supporting SOCK_CLOEXEC

httplib.h

@@ -6484,7 +6484,15 @@ inline bool Server::listen_internal() {
 #ifndef _WIN32
       }
 #endif
+
+#if defined _WIN32
+      // sockets conneced via WASAccept inherit flags NO_HANDLE_INHERIT, OVERLAPPED
+      socket_t sock = WSAAccept(svr_sock_, nullptr, nullptr, nullptr, 0);
+#elif defined __linux__
+      socket_t sock = accept4(svr_sock_, nullptr, nullptr, SOCK_CLOEXEC);
+#else
       socket_t sock = accept(svr_sock_, nullptr, nullptr);
+#endif
 
       if (sock == INVALID_SOCKET) {
         if (errno == EMFILE) {