Fix: discard event/lost packet counters

For per-pid buffers, we need to sum the counters for each application.

For per-uid buffers, if no application has launched yet, it should not
be considered as an error (which stops iteration on all other channels),
but rather as values of 0.

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