pgsql: Use mmap MAP_NOSYNC option to limit shared memory writes

Use mmap MAP_NOSYNC option to limit shared memory writes

mmap() is rarely used for shared memory, but when it is, this option is
useful, particularly on the BSDs.

Patch by Sean Chittenden

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/34afbba84e8fd12bbf905d68d1112f51ed584df9

Modified Files
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src/backend/storage/ipc/dsm_impl.c |    4 ++--
src/include/portability/mem.h      |    8 ++++++++
2 files changed, 10 insertions(+), 2 deletions(-)