timeutils: Support Tzif3 format timezone files too

The latest tzdata release added a new tzif version, with some minor
changes. This patch adds support for the format to our zone info parser,
and fixes #86.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #88 from algernon/f/tzfile3

timeutils: Support Tzif3 format timezone files too