Add event notifier and event notifier enabler

Idea
====
The purpose of the event notifiers is to allow the session daemon to
react to events in the tracer. For example, the user will be able to
start or stop tracing on a session when a specific tracepoint is fired.

An event notifier is really similar to a regular event. The main
difference is that when the tracepoint is fired the action of the event
notifier is to notify the session daemon about it. This mechanism will
use a special purpose ring buffer to expose these notifications to
userspace.

Unlike regular events, there are no claim on the timeliness of such
notifications.

Implementation
==============
This commit adds structures and functions related to event notifiers
mimicking what we currently do with regular events.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I962e6c7051693d6e4a79f89758f8bf1ebda6c148