Improve defaults shown in postgresql.conf.sample and pg_settings

Previously, these showed unlikely default values.  The new default value
128MB (since PG 10) is not always accurate since initdb tries several
increasing values, but it likely to be accurate.

Reported-by: Zhangjie <zhangjie2@fujitsu.com>

Discussion: https://postgr.es/m/TYWPR01MB7678772FD8640C404F1DC882F9079@TYWPR01MB7678.jpnprd01.prod.outlook.com

Author: Zhangjie

Backpatch-through: master

src/backend/utils/misc/guc.c

@@ -2333,7 +2333,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS
 		},
 		&NBuffers,
-		1024, 16, INT_MAX / 2,
+		16384, 16, INT_MAX / 2,
 		NULL, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -124,7 +124,7 @@
 
 # - Memory -
 
-#shared_buffers = 32MB			# min 128kB
+#shared_buffers = 128MB			# min 128kB
 					# (change requires restart)
 #huge_pages = try			# on, off, or try
 					# (change requires restart)

src/bin/initdb/initdb.c

@@ -1071,7 +1071,7 @@ setup_config(void)
 	else
 		snprintf(repltok, sizeof(repltok), "shared_buffers = %dkB",
 				 n_buffers * (BLCKSZ / 1024));
-	conflines = replace_token(conflines, "#shared_buffers = 32MB", repltok);
+	conflines = replace_token(conflines, "#shared_buffers = 128MB", repltok);
 
 #ifdef HAVE_UNIX_SOCKETS
 	snprintf(repltok, sizeof(repltok), "#unix_socket_directories = '%s'",