lock sessions

Added

• MQTT message on topic systemctl/hostname/lock-all-sessions
  instructs all sessions to activate screen locks
  (functionally equivalent to command loginctl lock-sessions)
• command line option --poweroff-delay-seconds (default: 4 seconds)

Changed

• docker image:
  • upgrade paho-mqtt to no longer suppress exceptions occuring in mqtt callbacks
    ( https://github.com/eclipse/paho.mqtt.python/blob/v1.5.1/ChangeLog.txt#L4 )
  • build stage: revert user after applying chown workaround for inter-stage copy