Fix: race between kconsumerd and sessiond on tear down

v2: minimize indentation by using return on condition.

Kconsumerd and sessiond both have reference on lttng-module. This can lead to a race
on modprobe_remove_lttng_all which might fail to unload modules due to
certain modules not having a ref count equal to zero at the time.

waitpid is used to force a synchronization on the child (kconsumer) termination.

This also have been applied to ust consumers for the sake of consistency.

Fixes: #878
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>