lib: remove bt_graph_interrupt, add bt_graph_borrow_default_interrupter

lib: remove bt_graph_interrupt, add bt_graph_borrow_default_interrupter

It is currently possible to interrupt a running graph with
bt_graph_interrupt.  It is however not possible to reset the default
interrupter that this function sets and resume the graph execution.

Rather than add a new graph function to reset the default graph
interrupter, introduce a new function,
bt_graph_borrow_default_interrupter, to borrow that default interrupter.
All the bt_interrupter API is therefore accessible with this default
interrupter.

This patch removes the bt_graph_interrupt function, since it's no longer
needed.

Change-Id: I277e6c8bb4e1be0a6557a6287b7ba8997e20d27b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2708
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>