babeltrace-cfg: use --params to set parameters of latest instance

babeltrace-cfg: use --params to set parameters of latest instance

Instead of:

    -i plugin.compcls:param=value,param2=value2

Do:

    -i plugin.compcls --params param=value,param2=value2

-p is now the short option of --params, and -P becomes the short
option of --plugin-path.

This stateful parsing will help when implementing default parameters
and adding other per-instance options.

This patch also makes the help message more readable.

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