Fix: checking for existing session daemon is done after daemonizing

Fix: checking for existing session daemon is done after daemonizing

The session daemon checks that no other session daemons are
running only after daemonizing. This means that launching the
deamon in background or daemon modes will appear to succeed even
if the launch failed due to an already present daemon.

The check is performed using both the client socket and the lock
file. This fix also addresses another problem that would cause
the pid file to be overwritten and deleted even if the session daemon
failed to launch.

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