Make shared_memory_size a preset option
bd17880 set up that as a memory parameter, but the docs told a different
story. A preset parameter is adapted here, as this option is compiled
at startup time.
Reported-by: Fujii Masao
Discussion: https://postgr.es/m/
4cc5b434-b174-9aae-197b-
737db6cac4e3@oss.nttdata.com