BUG/MINOR: log: missing timezone on iso dates.

The function timeofday_as_iso_us adds now the trailing local timezone offset.
Doing this the function could be use directly to generate rfc5424 logs.

It affects content of a ring if the ring's format is set to 'iso' and 'timed'.
Note: the default ring 'buf0' is of type 'timed'.

It is preferable NOT to backport this to stable releases unless bugs are
reported, because while the previous format is not correct and the new
one is correct, there is a small risk to cause inconsistencies in log
format to some users who would not expect such a change in a stable
cycle.
diff --git a/src/sink.c b/src/sink.c
index 19a555e..e273df4 100644
--- a/src/sink.c
+++ b/src/sink.c
@@ -176,7 +176,7 @@
 
 	if (sink->fmt == SINK_FMT_ISO || sink->fmt == SINK_FMT_TIMED) {
 		pfx[npfx].ptr = timeofday_as_iso_us(1);
-		pfx[npfx].len = 27;
+		pfx[npfx].len = 33;
 		npfx++;
 		goto send;
         }
diff --git a/src/time.c b/src/time.c
index a586e49..a6cfa8e 100644
--- a/src/time.c
+++ b/src/time.c
@@ -31,7 +31,7 @@
 static volatile unsigned long long global_now; /* common date between all threads (32:32) */
 
 static THREAD_LOCAL unsigned int iso_time_sec;     /* last iso time value for this thread */
-static THREAD_LOCAL char         iso_time_str[28]; /* ISO time representation of gettimeofday() */
+static THREAD_LOCAL char         iso_time_str[34]; /* ISO time representation of gettimeofday() */
 
 /*
  * adds <ms> ms to <from>, set the result to <tv> and returns a pointer <tv>
@@ -267,25 +267,36 @@
  * format. It uses a thread-local static variable that the reader can consume
  * for as long as it wants until next call. Thus, do not call it from a signal
  * handler. If <pad> is non-0, a trailing space will be added. It will always
- * return exactly 26 or 27 characters (depending on padding) and will always be
- * zero-terminated, thus it will always fit into a 28 bytes buffer.
+ * return exactly 32 or 33 characters (depending on padding) and will always be
+ * zero-terminated, thus it will always fit into a 34 bytes buffer.
+ * This also always include the local timezone (in +/-HH:mm format) .
  */
 char *timeofday_as_iso_us(int pad)
 {
 	struct timeval new_date;
 	struct tm tm;
-
+	const char *offset;
+	char c;
 	gettimeofday(&new_date, NULL);
 	if (new_date.tv_sec != iso_time_sec || !new_date.tv_sec) {
 		get_localtime(new_date.tv_sec, &tm);
-		if (unlikely(strftime(iso_time_str, sizeof(iso_time_str), "%Y-%m-%dT%H:%M:%S.000000", &tm) != 26))
-			strcpy(iso_time_str, "YYYY-mm-ddTHH:MM:SS.000000"); // make the failure visible but respect format.
+		offset = get_gmt_offset(new_date.tv_sec, &tm);
+		if (unlikely(strftime(iso_time_str, sizeof(iso_time_str), "%Y-%m-%dT%H:%M:%S.000000+00:00", &tm) != 32))
+			strcpy(iso_time_str, "YYYY-mm-ddTHH:MM:SS.000000-00:00"); // make the failure visible but respect format.
+		iso_time_str[26] = offset[0];
+		iso_time_str[27] = offset[1];
+		iso_time_str[28] = offset[2];
+		iso_time_str[30] = offset[3];
+		iso_time_str[31] = offset[4];
 		iso_time_sec = new_date.tv_sec;
 	}
+	/* utoa_pad adds a trailing 0 so we save the char for restore */
+	c = iso_time_str[26];
 	utoa_pad(new_date.tv_usec, iso_time_str + 20, 7);
+	iso_time_str[26] = c;
 	if (pad) {
-		iso_time_str[26] = ' ';
-		iso_time_str[27] = 0;
+		iso_time_str[32] = ' ';
+		iso_time_str[33] = 0;
 	}
 	return iso_time_str;
 }