cli: make config cli args functions return a status enum

cli: make config cli args functions return a status enum

The retcode returned by functions that create a bt_config from CLI args
is not very intuitive.  Replace that by a proper status enum.  Change
those functions to return the status as their return value, and return
the bt_config through an out parameter (only if the returned status is
BT_CONFIG_CLI_ARGS_STATUS_OK).

Change-Id: I2f273a6e28bcd0322a1b7ddd303639a696043872
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6278
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>