Re: Shmem queue is not flushed if receiver is not yet attached

I think that this patch is basically correct, except that it's not
correct to set mqh_counterparty_attached when receiver is still NULL.
Here's a v2 where I've attempted to correct that while preserving the
essence of your proposed fix.

I'm not sure that we need a shm_mq_flush(), but we definitely don't
have one currently, so I've also adjusted your patch to remove the
dead prototype.