Copy cur's snap_time to last when refreshing statistics. Fixes problem

where MB/s and tps statistics would always be zero, presumably because
they were being averaged out over the time between now and when the
system booted instead of a few seconds.

PR:		58683

usr.bin/systat/iostat.c

@@ -148,6 +148,8 @@ fetchiostat()
 	last.dinfo = cur.dinfo;
 	cur.dinfo = tmp_dinfo;
 
+	last.snap_time = cur.snap_time;
+         
 	/*
 	 * Here what we want to do is refresh our device stats.
 	 * getdevs() returns 1 when the device list has changed.