Add code from the inih library under libconfig

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add libconfig implementation and tests

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Load session daemon options from configuration file

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