xfce4-session: Backport lock command setting in xflock4
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
authored by Gonzalo Andreani and committed by Yegor Timoshenko c9c1ccda ab754a40