the new sensors tre can give us the number of sensors per type.

With this patch, we give up without bothering sysctl
kern_sensors.c::sensor_find() unless we know for sure that timedelta
sensor is present.
From: "Constantine A. Murenin" <cnst+openbsd@bugmail.mojo.ru>

src/usr.sbin/ntpd/sensors.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: sensors.c,v 1.30 2007/01/15 08:19:11 otto Exp $ */
+/*	$OpenBSD: sensors.c,v 1.31 2007/01/15 19:58:49 henning Exp $ */
 
 /*
  * Copyright (c) 2006 Henning Brauer <henning@openbsd.org>
@@ -76,6 +76,10 @@ sensor_probe(int devid, char *dxname, struct sensor *sensor)
 			log_warn("sensor_probe sysctl");
 		return (0);
 	}
+
+	if (sensordev.maxnumt[SENSOR_TIMEDELTA] == 0)
+		return (0);
+
 	strlcpy(dxname, sensordev.xname, MAXDEVNAMLEN);
 
 	slen = sizeof(*sensor);