pgsql: Improve defaults shown in postgresql.conf.sample and pg_settings

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: /messages/by-id/TYWPR01MB7678772FD8640C404F1DC882F9079@TYWPR01MB7678.jpnprd01.prod.outlook.com

Author: Zhangjie

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7bda63a487c542949c8150de8e63bc728e5e31e

Modified Files
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src/backend/utils/misc/guc.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
src/bin/initdb/initdb.c | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)