rsockets: Define options specific to rsockets

Allow a user to control some of the RDMA related attributes
of an rsocket through setsockopt/getsockopt.  A user specifies
that the rsocket should be modified through SOL_RDMA level.

This patch provides the initial framework.  Subsequent patches
will add the configurable parameters.

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