pgsql: Add shared_memory_type GUC.

Add shared_memory_type GUC.

Since 9.3 we have used anonymous shared mmap for our main shared memory
region, except in EXEC_BACKEND builds. Provide a GUC so that users
can opt for System V shared memory once again, like in 9.2 and earlier.

A later patch proposes to add huge/large page support for AIX, which
requires System V shared memory and provided the motivation to revive
this possibility. It may also be useful on some BSDs.

Author: Andres Freund (revived and documented by Thomas Munro)
Discussion: https://postgr.es/m/HE1PR0202MB28126DB4E0B6621CC6A1A91286D90%40HE1PR0202MB2812.eurprd02.prod.outlook.com
Discussion: https://postgr.es/m/2AE143D2-87D3-4AD1-AC78-CE2258230C05%40FreeBSD.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f1bebef60ec8f557324cd3bfc1671da1318de968

Modified Files
--------------
doc/src/sgml/config.sgml | 25 ++++++++++++++++
doc/src/sgml/runtime.sgml | 17 +++++++----
src/backend/port/sysv_shmem.c | 42 ++++++++++-----------------
src/backend/storage/ipc/ipci.c | 2 ++
src/backend/utils/misc/guc.c | 23 +++++++++++++++
src/backend/utils/misc/postgresql.conf.sample | 6 ++++
src/include/storage/pg_shmem.h | 19 +++++++++++-
7 files changed, 101 insertions(+), 33 deletions(-)