patch: add MAP_HUGETLB to mmap() where supported (WIP)

The attached patch adds the MAP_HUGETLB flag to mmap() for shared memory
on systems that support it. It's based on Christian Kruse's patch from
last year, incorporating suggestions from Andres Freund.

On a system with 4GB shared_buffers, doing pgbench runs long enough for
each backend to touch most of the buffers, this patch saves nearly 8MB of
memory per backend and improves performances by just over 2% on average.

It is still WIP as there are a couple of points that Andres has pointed
out to me that haven't been addressed yet; also, the documentation is
incomplete.

Richard

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Richard Poole                 http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services