Support TCP keep alive

[iislucas]

TCP socket APIs typically have a keep alive option (e.g. see: http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html). This is often a helpful way to know when an application has been disconnected from a server. e.g. it would help fix: UWNetworksLab/uProxy-p2p#270

Proposal here is that freedom TCP API supports keepalive.