TCP_KEEPINTVL should be 1 second

Currently we set it to 5 minutes, matching TCP_KEEPIDLE, but that seems incorrect. KEEPINTVL is how frequently to probe a connection that has been idle for KEEPIDLE seconds until we get a successful reply from the remote host, which restarts the KEEPIDLE timer. Essentially this is used to detect blackholed connections, and there is no reason to wait 45 minutes (default value 9 of TCP_KEEPCNT * KEEPINTVL) to realize a connection is dead. This will lower it to 5 minutes + 10 seconds.