Move CLI params parsing to its own lib

Move CLI params parsing to its own lib

This will allow using it for parameter parsing in a test, for
convenience.

The sharp-eyed reader will have noticed that this patch also removes
unused structures from src/cli/babeltrace2-cfg-cli-args.c.  These were
likely forgotten there when the parameter parsing code was moved to its
own file.

Change-Id: Ibc7ecb238905051492718f1d176c0fb5f0c172a8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2135
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>