sessiond: kernel triggers: add infrastructure to create event notifiers

sessiond: kernel triggers: add infrastructure to create event notifiers

Add the infrastructure to initialize the kernel tracer event notifier
group and individual event notifiers from event rules issued from
triggers.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I20127f655018260e45566d09d11c2852cd3b3f97
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479