pg_rewind: Add dbname to primary_conninfo when using --write-recovery-conf.

pg_rewind: Add dbname to primary_conninfo when using --write-recovery-conf.

This commit enhances pg_rewind's --write-recovery-conf option to
include the dbname in the generated primary_conninfo value when
specified in the --source-server option. With this modification, the
rewound server can connect to the primary server without manual
configuration file modifications when sync_replication_slots is
enabled.

Reviewed-by: Hayato Kuroda 
Reviewed-by: Peter Smith 
Discussion: https://postgr.es/m/CAD21AoAkW=Ht0k9dVoBTCcqLiiZ2MXhVr+d=j2T_EZMerGrLWQ@mail.gmail.com