Fix: Possible memory leak when multiple config files are loaded

Some configuration options could leak when initialized multiple times
from different configuration files and from the command line arguments.
The previous options are now freed' before being set.

Fixes #796

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>