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

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.