Support mkosi ssh for multiple running instances of the same image

Support mkosi ssh for multiple running instances of the same image

Let's add a stopgap solution until systemd-machined supports everything
we need. We maintain a super basic JSON state file in the runtime directory
that is used to map a machine name to the corresponding SSH proxy command.

We also store the path to the ssh key in there so that mkosi ssh can be
run from every directory.

The new Machine= option allows selecting the machine name to use. Unless
set explicitly, we also use the machine name as the hostname for the machine.