In timegm() initialize tm_isdst to 0 like tzcode does.

lib/util/timegm.c

@@ -88,6 +88,7 @@ sudo_timegm(struct tm *tm)
 {
     time_t result;
 
+    tm->tm_isdst = 0;
     result = mktime(tm);
     if (result != -1)
 	result += get_gmtoff(&result);