Implement 2-step registration of userspace probe events

Userspace-probe registration is implemented in a 2-step process. The two
steps registration is necessary because a userspace-probe event can have
an arbitrary number of call sites.

The first step is registering the event through the existing
LTTNG_KERNEL_EVENT command on the channel ioctl FD. This creates an
event with no callsites and thus no instrumentation at all.

The second step is attaching callsites to the event. It's done through
the new LTTNG_KERNEL_ADD_CALLSITE command on the event ioctl FD
received during the first step.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>