[ofw] RDMA-cm completion channel api

[Sean Hefty]

>struct rdma_event_channel *rdma_create_event_channel(void);
>void rdma_destroy_event_channel(struct rdma_event_channel *channel);
>int rdma_get_cm_event(struct rdma_event_channel *channel,
>       struct rdma_cm_event **event);
>__declspec(dllexport)
>int rdma_ack_cm_event(struct rdma_cm_event *event);
>
>I'm trying to figure out how can an rdma_get_cm_event() be canceled
>when running over windows.

We can probably modify destroy to cancel the operation.

Currently, the only way is to use the underlying COMP_CHANNEL structure.
(This is similar to accessing the underlying fd on linux.)  This is how
DAPL handles this.

>This will be even more important once we will implement rdma_cm in
>kernel (in which one can not press ctrl c).

A kernel version could use a callback model to report events, rather
than a polling model.  (This would be similar to linux as well.)

- Sean