repmgr standby switchover: add "%p" event notification parameter

This will contain the node ID of the former primary.

dbutils.h

@@ -174,11 +174,13 @@ typedef struct s_event_info
 {
 	char	   *node_name;
 	char	   *conninfo_str;
+	int			former_primary_id;
 } t_event_info;
 
 #define T_EVENT_INFO_INITIALIZER { \
 	NULL, \
- 	NULL \
+	NULL, \
+	UNKNOWN_NODE_ID \
 }