/*D
   MPIX_Async_start - [short description]

Synopsis:
.vb
int MPIX_Async_start(MPIX_Async_poll_function *poll_fn, void *extra_state,
                     MPIX_Stream stream)
.ve

Input Parameters:
+ poll_fn - user defined poll function for progressing async things (function)
. extra_state - extra state for callback function (None)
- stream - stream object (handle)

.N ThreadSafe

.N Fortran

.N Errors
.N MPI_SUCCESS
.N MPIX_ERR_STREAM
.N MPI_ERR_ARG
.N MPI_ERR_OTHER

D*/

