When systemctl --user restart fails during 'hermes update' or
'hermes gateway restart', check linger status and tell the user
exactly what to run (sudo -S -p '' loginctl enable-linger) instead of
silently falling back to foreground mode.
Also applies _ensure_user_systemd_env() to the raw systemctl calls
in cmd_update so they work properly on SSH sessions where D-Bus
env vars are missing.
e323e2f876