Make kernel tracer version global to the session daemon

A version check is already performed during the initialization of
the kernel tracer. This patch makes the result available as a
global variable in main.c in order to check for support of
the kernel tracer commands required to support the rotation of
sessions.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>