cli, tests: rename "leftover" terms to "non-option"

cli, tests: rename "leftover" terms to "non-option"

"Leftover" is a popt term, and we don't use popt anymore. Arguments
which are not options are not leftover anymore: they are treated within
the same loop as option arguments, and their position relative to option
arguments are important, so we simply call them "non-option arguments".

Note that "non-option" is also a term that getopt(1) and getopt(3) use.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6c27e6a18f9daa4a9d2e5f79058a94c926102da3
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1817
Tested-by: jenkins <jenkins@lttng.org>