The current code is not compatible with current dkr protocols anyway,
and dkr has a different focus ("microservices") than nspawn anyway
("whole machine containers"), hence drop support for it, we cannot
reasonably keep this up to date, and it creates the impression we'd
actually care for the microservices usecase.
"machinectl list" only lists running machines while many of the MACHINES
commands use names of images; both running and non-running.
List machines from both "list" and "list-images" and use sort -u to
avoid duplicates.
