Fix: Replace bt_timegm with a thread-safe implementation

Fix: Replace bt_timegm with a thread-safe implementation

The current compat wrapper for timegm is not thread-safe, it modifies
the process wide time settings with tzset. This was not a problem in bt1
as it was only used in the cli binary but it is now used in the trimmer
plugin which is used by the library.

Replace the wrapper with a basic implementation and add more tests.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>