Improve xuserrun

- Used bash home directory expansion instead of hard-coding
  /home into the script.
- Silence show-session errors for unknown attributes in some systemd
  versions.
- Use list-sessions instead of show-seat to discover sessions (which
  is broken in recent systemd versions).
- Look for sessions with 'Class=user' to avoid picking login managers
  (unless there is no user logged-in).