BUG/MINOR: log: GMT offset not updated when entering/leaving DST

GMT offset used in local time formats was computed at startup, but was not updated when DST status changed while running.

For example these two RFC5424 syslog traces where emitted 5 seconds apart, just before and after DST changed:
  <14>1 2016-03-27T01:59:58+01:00 bunch-VirtualBox haproxy 2098 - - Connect ...
  <14>1 2016-03-27T03:00:03+01:00 bunch-VirtualBox haproxy 2098 - - Connect ...

It looked like they were emitted more than 1 hour apart, unlike with the fix:
  <14>1 2016-03-27T01:59:58+01:00 bunch-VirtualBox haproxy 3381 - - Connect ...
  <14>1 2016-03-27T03:00:03+02:00 bunch-VirtualBox haproxy 3381 - - Connect ...

This patch should be backported to 1.6 and partially to 1.5 (no fix needed in log.c).
diff --git a/include/common/standard.h b/include/common/standard.h
index 9abdb06..353d0b0 100644
--- a/include/common/standard.h
+++ b/include/common/standard.h
@@ -860,9 +860,6 @@
 
 extern const char *monthname[];
 
-/* numeric timezone (that is, the hour and minute offset from UTC) */
-char localtimezone[6];
-
 /* date2str_log: write a date in the format :
  * 	sprintf(str, "%02d/%s/%04d:%02d:%02d:%02d.%03d",
  *		tm.tm_mday, monthname[tm.tm_mon], tm.tm_year+1900,
@@ -873,6 +870,12 @@
  */
 char *date2str_log(char *dest, struct tm *tm, struct timeval *date, size_t size);
 
+/* Return the GMT offset for a specific local time.
+ * The string returned has the same format as returned by strftime(... "%z", tm).
+ * Offsets are kept in an internal cache for better performances.
+ */
+const char *get_gmt_offset(struct tm *tm);
+
 /* gmt2str_log: write a date in the format :
  * "%02d/%s/%04d:%02d:%02d:%02d +0000" without using snprintf
  * return a pointer to the last char written (\0) or