Fix pgpool_setup problem with -r option and PostgreSQL 12.

Fix pgpool_setup problem with -r option and PostgreSQL 12.

pgpool_setup -r uses pg_rewind if it's possible. After rewinding,
pgpool_setup failed to create standby.signal file, which is required in
PostgreSQL 12 or later, on the recovered target node. As a result, the
recovered node booted up as a primary server, rather than a standby
server.

Discussion: https://www.pgpool.net/mantisbt/view.php?id=624