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

src/backend/utils/misc/guc.c

@@ -2339,7 +2339,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+		{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
 			NULL,
 			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB