When dealing with tevent, we use EAGAIN to say that an asynchronous request continues and EOK when it's done. Sometimes the asynchronous request is not even started and we need to mark a tevent_req as finished (tevent_req_done()) before any callback is attached to it. To make sure the callback is executed in this case, tevent_req_post() is called.

Both tevent_req_done() and tevent_req_post() should be called only from _send() for better clarity.

Many sdap_nested_group* functions return wrong error codes, they either return correctly EOK or they return EOK instead of EAGAIN. However, in both situations, the value EOK is usually translated to EAGAIN in the caller.

This forces us to use tevent_req_done/post() deep in the call hierarchy, outside _send() function. We should rewrite the return codes to obey this rule.