Allow to run pgpool_setup on PostgreSQL 15.

Allow to run pgpool_setup on PostgreSQL 15.

basebackup.sh created by pgpool_setup uses pg_start_backup() and
pg_stop_backup().  PostgreSQL 15 renamed them. But there's more
problem. The renamed functions (pg_backup_start() and
pg_backup_stop()) must be called within a same connection. Adapting
these requires non-trivial changes. So I decided to rewrite the code
to not use pg_start_backup() and pg_stop_backup() so that it uses
pg_basebackup command instead.

Per https://www.pgpool.net/mantisbt/view.php?id=757