xss-lock: use fork with session param support
xss-lock: use fork with session param support

xss-lock uses current session that cannot be overriden. This prevents starting
xss-lock as systemd user service, as described here:
https://bitbucket.org/raymonad/xss-lock/issues/13/allow-operation-as-systemd-user-unit

This change uses source from github.com/xdbob/xss-lock that adds `--session`
parameter.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs