Enable TCP keep-alives on external connections
Enable TCP keep-alives on external connections

We wish to detect when connections have disappeared (e.g. when the
host network environment changes) and free any associated filehandles.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit