cli: apply log levels (`--log-level` option) to leftovers

cli: apply log levels (`--log-level` option) to leftovers

It is currently not possible to apply --log-level after a leftover.

This patch proposes a way to allow it and to deal with the ambiguity
that it poses.

In the simple case, we have:

    babeltrace2 my-traces --log-level=TRACE

All source components auto-discovered from the `my-traces` leftover will
have the TRACE log level.

If we have more than one leftover, but _no_ cross-leftover grouping,
then it is also intuitive:

    babeltrace2 my-traces-1 --log-level=TRACE my-traces-2 --log-level=DEBUG

... all source components discovered from `my-traces-1` will have log
level TRACE and all source components discovered from `my-traces-2` will
have log level DEBUG.

It becomes less obvious when components are given inputs coming from
multiple leftovers (because of the auto-discovery grouping feature):
which log level do they receive?  For example, if the following line:

    babeltrace2 my-traces-1 --log-level=TRACE my-traces-2 --log-level=DEBUG

leads to these components getting instantiated, with these inputs:

 * Source component X with inputs `my-traces-1/x` and `my-traces-2/x`.
 * Source component Y with input `my-traces-1/y`

In this case, each component receives (on the `run` command line) the
log level options of all leftovers that contributed to its inputs, in
the same order as they are provided on the command line.  The resulting
`run` command line for the example above could therefore look like:

    ... --component x:src.my.comp --log-level=TRACE --log-level=DEBUG \
        --component y:src.my.comp --log-level=TRACE

resulting in these effective log levels:

 * Source component X: log level DEBUG
 * Source component Y: log level TRACE

Change-Id: I5e1bf9e1b4dd139ff7900d81b302a1eda72fb37f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1810
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>