Turn on SO_KEEPALIVE for cloud connections.

This is not totally redundant.  It's true the cloud protocol has
something like keepalives of its own, but they don't kick in until the
peer connection comes fully up.  Connections can still wedge while
doing the initial crypto exchange, so keepalives are desirable.  (I
could add timeouts to the crypto exchange, true.  But there's always
the risk that I missed something; keepalives cover for all such cases.)