units: make graphical.target dependencies more complete and similar to those of multi-user.target
units: rework systemd-nspawn@.service unit

- Unescape instance name so that we can take almost anything as instance
  name.

- Introduce "machines.target" which consists of all enabled nspawns and
  can be used to start/stop them altogether

- Look for container directory using -M instead of harcoding the path in
  /var/lib/container
machinectl: add new "start" verb to start a container as a service in nspawn
machinectl: add "enable" and "disable" verbs for enabling/disabling systemd-nspawn for containers

This is basically just a shortcut for "systemctl enable
systemd-nspawn@<foobar>.service", but does escaping.
Update TODO