screensaver: Prevent recursive calling of lock command
This is not intended to prevent a malicious attack but an accidental recursive call. When xfce4-session!41 (merged) is merged, this will allow in particular to address xfce4-session#120 (closed) in this sense. The original idea is from Jarno Suni in xfce4-session!20 (closed).
Edited by Gaël Bonithon