pgsql: Add pg_shmem_allocations view.

Add pg_shmem_allocations view.

This tells you about allocations that have been made from the main
shared memory segment. The original patch also tried to show information
about dynamic shared memory allocation as well, but I decided to
leave that problem for another time.

Andres Freund and Robert Haas, reviewed by Michael Paquier, Marti
Raudsepp, Tom Lane, Álvaro Herrera, and Kyotaro Horiguchi.

Discussion: http://postgr.es/m/20140504114417.GM12715@awork2.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ed10f32e37e9a16814c25e400d7826745ae3c797

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 85 ++++++++++++++++++++++++++++
doc/src/sgml/xfunc.sgml | 2 +-
src/backend/catalog/system_views.sql | 6 ++
src/backend/storage/ipc/shmem.c | 106 ++++++++++++++++++++++++++++++++++-
src/include/catalog/pg_proc.dat | 9 +++
src/include/storage/shmem.h | 3 +-
src/test/regress/expected/rules.out | 5 ++
7 files changed, 212 insertions(+), 4 deletions(-)