Configurable kernel module probes support

Create a kernel module probes list and use it to load the specified
probes. The probes are selectable by the --kmod-probes command line
option to lttng-sessiond or the LTTNG_KMOD_PROBES environment variable.
If neither is set all probes are loaded so the current behaviour is not
changed.

Signed-off-by: Jan Glauber <jan.glauber@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>