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.

log.h

@@ -14,6 +14,8 @@
 #define OM_COMMAND_LINE 1
 #define OM_DAEMON		2
 
+#define DEFAULT_LOG_STATUS_INTERVAL 300
+
 extern void
 stderr_log_with_level(const char *level_name, int level, const char *fmt,...)
 __attribute__((format(PG_PRINTF_ATTRIBUTE, 3, 4)));