shm_mq fix for non-blocking mode

The shm_mq code handles blocking mode and non-blocking mode
asymmetrically in a couple of places, with the unfortunate result that
if you are using non-blocking mode, and your counterparty dies before
attaching the queue, operations on the queue continue to return
SHM_MQ_WOULD_BLOCK instead of, as they should, returning
SHM_MQ_DETACHED. The attached patch fixes the problem. Thanks to my
colleague Rushabh Lathia for helping track this down.

(There's are some further bugs in this area outside the shm_mq code
... but I'm still trying to figure out exactly what they are and what
we should do about them. This much, however, seems clear-cut.)

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company