babeltrace(1): handle SIGINT to cancel the graph gracefully

babeltrace(1): handle SIGINT to cancel the graph gracefully

It goes like this:

1. Ctrl+C
2. SIGINT
3. sigint_handler()
4. Cancel the application's graph.
5. bt_graph_run() eventually returns BT_GRAPH_STATUS_CANCELED or
   BT_GRAPH_STATUS_AGAIN.
6. When we get those statuses, we check if the graph is canceled and
   quit the loop if so.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>