Fix: add -b, --background option

This was removed during a merge from master by mistake.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Update version to v2.4.0-rc4

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: memory leak when UST events are enabled with exclusions

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>