Enable keep-alive packets

When the timeout is not specified，the TCP connection will be dead if the connection is terminated unexpectedly.

Because Mac OS X does not have the TCP_KEEPINTVL socket parameter that the interval between packets sent to validate be determined by OS.
<del>Based on my observations, the value is 10 minutes.</del>

Default:
TCP_KEEPINTVL = net.inet.tcp.keepintvl = 75000ms
TCP_KEEPCNT = 8