Fix: use MT-safe strtok_r in multithreaded context

Fix: use MT-safe strtok_r in multithreaded context

append_list_to_probes uses the non-multithread-safe "strtok" while in
multithreaded context.

It is called by modprobe_lttng_data(), which is called from
init_kernel_tracer().

init_kernel_tracer is used from main() after other threads are created,
and also from process_client_msg() when a user attempts to interact with
the kernel domain if the kernel tracer has not yet been successfully
initialized.

Fixed by using the MT-safe strtok_r() instead.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ic3f81551b8508451e0f9733c5de56a4d4407b813