repmgrd: improve log output

- emit explicit startup NOTICE
- emit NOTICE when falling back to degraded monitoring on a primary node
- improve log message and event notification details when monitoring
  a former primary which has been reconnected as a standby

repmgrd.c

@@ -254,6 +254,8 @@ main(int argc, char **argv)
 		strncpy(config_file_options.log_level, cli_log_level, MAXLEN);
 	}
 
+	log_notice(_("repmgrd (repmgr %s) starting up"), REPMGR_VERSION);
+
 	/*
 	 * -m/--monitoring-history, if provided, will override repmgr.conf's
 	 * monitoring_history; this is for backwards compatibility as it's