Increase sensor polling interval to 30s, just like ntp polls. This

improves sensor timekeeping significantly.
Before this patch my test system's frequency adjustment would range
between -350 and +250, with timedelta rarely getting close to 0. After,
frequency adjustmens is on the order of +/- 0.05ppm, with time +/- a
few microseconds away from 0
ok henning, mbalmer, otto

src/usr.sbin/ntpd/ntpd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.h,v 1.82 2007/03/01 17:50:42 henning Exp $ */
+/*	$OpenBSD: ntpd.h,v 1.83 2007/03/23 14:22:40 ckuethe Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -64,7 +64,7 @@
 
 
 #define	SENSOR_DATA_MAXAGE	(15*60)
-#define	SENSOR_QUERY_INTERVAL	5
+#define	SENSOR_QUERY_INTERVAL	30
 #define	SENSOR_SCAN_INTERVAL	(5*60)
 
 enum client_state {