initial implementation of tuning TCP keep-alives for TCP socket

currently not fully cross-platform
work on linux
possibly work in *bsd
and could be enhanced to work on windows

src/tcp_listener.cpp

@@ -87,6 +87,7 @@ void zmq::tcp_listener_t::in_event ()
         return;
 
     tune_tcp_socket (fd);
+    tune_tcp_keepalives (fd, options.tcp_keepalive, options.tcp_keepalive_cnt, options.tcp_keepalive_idle, options.tcp_keepalive_intvl);
 
     //  Create the engine object for this connection.
     stream_engine_t *engine = new (std::nothrow) stream_engine_t (fd, options);