rsocket: Succeed setting SO_KEEPALIVE option

rsocket: Succeed setting SO_KEEPALIVE option

memcached sets SO_KEEPALIVE, so succeed any requests to set
that option.  We don't actually implement keepalive at this time.

To implement keepalive, we would need to record the last time
that a message was sent or received on an rsocket.  If no
new messages are processed within the keepalive timeout, then
we would need to issue a keepalive message.  For rsockets,
this would simply mean sending a 0-byte control message that
gets ignored on the remote side.

The only real difficulty with handlng keepalive is doing it
without adding threads.  This requires additional work in
poll to occasionally timeout, send keepalive messages, then
return to polling if no new data has arrived.  Alternatively,
we can add a thread to handle sending keepalive messages.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>