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

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).