Fix: application exit race with pthread cancel

Fix: application exit race with pthread cancel

Listener threads can be cancelled with ust lock held, which can hang the
following ust cleanup routine, because tracepoint probe unregister needs
to take the ust lock.

Fix this by disabling pthread cancellation for the entire duration of
the ust lock.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>