BUG/MINOR: log: Don't use strftime() which can clobber timezone if chrooted

BUG/MINOR: log: Don't use strftime() which can clobber timezone if chrooted

The strftime() function can call tzset() internally on some platforms.
When haproxy is chrooted, the /etc/localtime file is not found, and some
implementations will clobber the content of the current timezone.

The GMT offset is computed by diffing the times returned by gmtime_r() and
localtime_r(). These variants are guaranteed to not call tzset() and were
already used in haproxy while chrooted, so they should be safe.

This patch must be backported to 1.6 and 1.5.