Lists logged-in sessions (local and ssh), one entry per terminal,
including multiplexed ssh connections (ControlMaster). Needed because
Debian 13 no longer populates utmp, so w/who show nothing.
- Sessions and origin via systemd-logind (loginctl)
- Terminals detected via session leaders and cgroup mapping
- sudo shells (own pty) shown indented below their terminal
- BEFEHL column shows the current foreground process
- Sessions without a terminal (scp, sftp, ssh -N) are listed too
