actions: introduce lttng_action_init

This function is to be used to initialize the common portion of all
action structures.  I have updated the sole currently existing action
(notify), but the function will also be used in subsequent patches.

Change-Id: I4e42554c42a4e6a5ef2da292a6dfeb708d72a602
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>