Fix strflocaltime on DST values

The following behaviour demonstrates how the daylight savings value gets
lost when a time value gets turned into an internal array of numbers and
then back into a (struct tm):

        $ TZ=Europe/London ./jq -r -n '1504803635 | strflocaltime("%Y-%m-%d %H:%M:%S (%Z)")'
        2017-09-07 18:00:35 (GMT)

After this change however, this works as expected:

        $ TZ=Europe/London ./jq -r -n '1504803635 | strflocaltime("%Y-%m-%d %H:%M:%S (%Z)")'
        2017-09-07 18:00:35 (BST)

src/builtin.c

@@ -1201,7 +1201,8 @@ static jv tm2jv(struct tm *tm) {
                   jv_number(tm->tm_min),
                   jv_number(tm->tm_sec),
                   jv_number(tm->tm_wday),
-                  jv_number(tm->tm_yday));
+                  jv_number(tm->tm_yday),
+                  jv_number(tm->tm_isdst));
 }
 
 #if defined(WIN32) && !defined(HAVE_SETENV)
@@ -1422,12 +1423,9 @@ static int jv2tm(jv a, struct tm *tm) {
   TO_TM_FIELD(tm->tm_sec,  a, 5);
   TO_TM_FIELD(tm->tm_wday, a, 6);
   TO_TM_FIELD(tm->tm_yday, a, 7);
+  TO_TM_FIELD(tm->tm_isdst, a, 8);
   jv_free(a);
 
-  // We use UTC everywhere (gettimeofday, gmtime) and UTC does not do DST.
-  // Setting tm_isdst to 0 is done by the memset.
-  // tm->tm_isdst = 0;
-
   // The standard permits the tm structure to contain additional members. We
   // hope it is okay to initialize them to zero, because the standard does not
   // provide an alternative.