Always explicitly set tm_isdst before calling mktime().

Except in rare cases, we want to set it to -1 so that we let mktime()
determine whether DST/Summer Time was in effect at the given date and
time rather than pretending that we know whether it's in effect or not.

Change-Id: I0ea75317dd308a515cedf4d1260b583e1592cc9b
Reviewed-on: https://code.wireshark.org/review/27431
Reviewed-by: Guy Harris <guy@alum.mit.edu>

editcap.c

@@ -1308,6 +1308,7 @@ main(int argc, char *argv[])
         stoptm.tm_year = 135;
         stoptm.tm_mday = 31;
         stoptm.tm_mon = 11;
+        stoptm.tm_isdst = -1;
 
         stoptime = mktime(&stoptm);
     }

ui/time_shift.c

@@ -244,6 +244,7 @@ time_string_to_nstime(const gchar *time_text, nstime_t *packettime, nstime_t *ns
     tm.tm_hour = h;
     tm.tm_min = m;
     tm.tm_sec = (int)floorl(f);
+    tm.tm_isdst = -1;
     tt = mktime(&tm);
     if (tt == -1) {
         return "Mktime went wrong. Is the time valid?";

wiretap/logcat_text.c

@@ -159,6 +159,7 @@ static void get_time(gchar *string, wtap_rec *rec) {
                     &date.tm_min, &date.tm_sec, &ms)) {
         date.tm_year = 70;
         date.tm_mon -= 1;
+        date.tm_isdst = -1;
         seconds = mktime(&date);
         rec->ts.secs = (time_t) seconds;
         rec->ts.nsecs = (int) (ms * 1e6);