Fix: Create a lock file to prevent multiple session daemons

A lock file is used to ensure that only one session daemon per UID is
running on the system. The lock file is created in the rundir.

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