Improve the lttng list feature

Improve the lttng list feature

Introduce three new functions to the public API which are
lttng_list_channels, lttng_list_events and lttng_list_domains.

Also add the lttng_list_kernel_events which list available tracing
events of the kernel.

Adds new options to the lttng list command for listing a lot more
details of a tracing session including channels, events and domains.

Also adds the enabled flag to the lttng_event and lttng_channel
structure.

Fix a bad pointer cast in list events.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>