util: when creating temporary filename for atomic creation of files, add an extra "#" to the name

That way, we have a simple, somewhat reliable way to detect such
temporary files, by simply checking if they start with ".#".
machined/machinectl: add logic to show list of available images

This adds a new bus call to machined that enumerates /var/lib/container
and returns all trees stored in it, distuingishing three types:

        - GPT disk images, which are files suffixed with ".gpt"
        - directory trees
        - btrfs subvolumes
verbs: when invoking the default verb, pass a faked argv array, with just the verb in it

That way the dispatcher calls know how they got called.
import: Verb[] array can be static, too
machinectl: port machinectl to new verbs logic
update TODO