shm_mq: Third attempt at fixing nowait behavior in shm_mq_receive.

Commit a1480ec1d3bacb9acb08ec09f22bc25bc033115b purported to fix the
problems with commit b2ccb5f4e6c81305386edb34daf7d1d1e1ee112a, but it
didn't completely fix them.  The problem is that the checks were
performed in the wrong order, leading to a race condition.  If the
sender attached, sent a message, and detached after the receiver
called shm_mq_get_sender and before the receiver called
shm_mq_counterparty_gone, we'd incorrectly return SHM_MQ_DETACHED
before all messages were read.  Repair by reversing the order of
operations, and add a long comment explaining why this new logic is
(hopefully) correct.

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