[patch] Imporve pqmq

[Xiaoran Wang]

Hi,
    When I use the 'pqmq' recently, I found some issues, just fix them.

     Allow the param 'dsm_segment *seg' to be NULL in function
    'pq_redirect_to_shm_mq'. As sometimes the shm_mq is created
    in shared memory instead of DSM.

    Add function 'pq_leave_shm_mq' to allow the process to go
    back to the previous pq environment.

0001-Improve-pqmq.patch
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