Add configuration option "log_status_interval"

Normally (outside of log level DEBUG) repmgrd doesn't generate any
kind of log output, so examining the log file may give the impression
it's not working. Output an informational message at regular intervals
to show it's up and running.

config.h

@@ -62,6 +62,7 @@ typedef struct
 	char		log_level[MAXLEN];
 	char		log_facility[MAXLEN];
 	char		log_file[MAXLEN];
+	int			log_status_interval;
 
 	/* standby clone settings */
 	bool		use_replication_slots;
@@ -116,7 +117,7 @@ typedef struct
 		/* node information */ \
 		UNKNOWN_NODE_ID, NO_UPSTREAM_NODE, "", "", "", "", REPLICATION_TYPE_PHYSICAL, \
 		/* log settings */ \
-		"", "", "", \
+		"", "", "", DEFAULT_LOG_STATUS_INTERVAL,	\
 		/* standby clone settings */ \
 		false, "", "", "", "", { NULL, NULL },	\
 		/* repmgrd settings */ \