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