Fix pcp_promote_node bug that fails promoting node 0

Fix pcp_promote_node bug that fails promoting node 0

The master node could not be promoted by pcp_promote_node with
the following error;

 FATAL: invalid pgpool mode for process recovery request
 DETAIL: specified node is already primary node, can't promote node id 0

In streaming replication mode, there is a case that Pgpool-II
regards the status of primary node as "standby" for some reasons,
for example, when pg_ctl promote is executed manually during
Pgpool-II is running, in which case, it seems to Pgpool-II
that the primary node doesn't exist.

This status mismatch should be fixe by pcp_promote_node, but when the node
is the master node (the first alive node), it fails as mentioned above.

The reason is as following. before changing the status, pcp_promote_node
checks if the specified node is already primary or not by comparing the
node id with PRIMARY_NODE_ID. However, if the primary doesn't exist from
Pgpool-II's view, PRIMARY_NODE_ID is set to 0, which is same as MASTER_NODE_ID.
Hence, when the master node is specified to be promoted, pcp_promote_node
is confused that this node is already primary and doesn't have to be
promoted, and it exits with the error.

To fix this, pcp_promote_node should check the node id by using
REAL_PRIMARY_NODE_ID, which is set -1 when the primary doesn't exist,
rather than PRIMARY_NODE_ID.