pgsql: "Pin", rather than "keep", dynamic shared memory mappings and se

"Pin", rather than "keep", dynamic shared memory mappings and segments.

Nobody seemed concerned about this naming when it originally went in,
but there's a pending patch that implements the opposite of
dsm_keep_mapping, and the term "unkeep" was judged unpalatable.
"unpin" has existing precedent in the PostgreSQL code base, and the
English language, so use this terminology instead.

Per discussion, back-patch to 9.4.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1c49dae165bcee69bf3327d6ae20271d82cdf6bf

Modified Files
--------------
src/backend/storage/ipc/dsm.c | 10 +++++-----
src/backend/storage/ipc/dsm_impl.c | 2 +-
src/include/storage/dsm.h | 4 ++--
src/include/storage/dsm_impl.h | 2 +-
4 files changed, 9 insertions(+), 9 deletions(-)