Add libkmod rmmod support

We previously had support to load modules with libkmod but we always
unload modules by calling the modprobe binary.

This patch adds the libkmod module unloading which is faster and gives
us better error handling. This implementation also graciously handles
built-in lttng kernel modules.

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