Clean-up: lttng: prepend `the_` to global variables in list.c

This avoids name clashes between the global variable `handle` and
function parameters of the same name. Change all global variables of the
file while at it.

Change-Id: Ice1a4f98270f439f5e5001014d9b3d6f2d8a5803
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>