pgsql: Extend dsm API with a new function dsm_unpin_segment.

Extend dsm API with a new function dsm_unpin_segment.

If you have previously pinned a segment and decide that you don't
actually want to keep it around until shutdown, this new API lets you
remove the pin. This is pretty trivial except on Windows, where it
requires closing the duplicate handle that was used to implement the
pin.

Thomas Munro and Amit Kapila, reviewed by Amit Kapila and by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0fda682e542c9acd368588e50a1993fecd3b73e2

Modified Files
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src/backend/storage/ipc/dsm.c | 117 +++++++++++++++++++++++++++++++++++--
src/backend/storage/ipc/dsm_impl.c | 57 +++++++++++++++++-
src/include/storage/dsm.h | 1 +
src/include/storage/dsm_impl.h | 4 +-
4 files changed, 169 insertions(+), 10 deletions(-)