Up until xfce 4.13, xflock4 only had support for "whitelisted" (*cough* hardcoded *cough*) locks screens. Version 4.13 added the ability for the user to specify the lock command via the xfce settings. This PR backports that functionality https://github.com/xfce-mirror/xfce4-session/blob/xfce4-session-4.13.0/scripts/xflock4