Cleanup: ust start/stop trace

Move setting/clearing the session "active" state from cmd.c to
cmd_start_trace()/cmd_stop_trace() for better encapsulation of
behavior.

Reduces the amount of code to maintain in the catch-all cmd.c.

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