logind: only allow one shutdown/sleep action to be queued at the same time

This should make sure that closing the lid while shutting down won't
suspend the machine but will simply cause the shutdown to complete.
logind: send Resumed() signal after we come back from suspend/hibernate/hybrid-sleep

This allows clients to get asynchronous notifications for user-requested
suspend/hibernate cycles. Kernel-triggered automatic suspending is not
covered.
logind: add UnlockSessions() clal to complement LockSessions()
update TODO
loginctl: show seat master and uaccess devices in seat-status output