Device-based gettys

Johan pointed out that the current one-file-per-getty configuration isn't very flexible, not to mention not very Upstartish; and there's a much better solution. Have a single getty job that spawns an instance for each terminal configured.

• Spawn instance, and set goal to start, on the tty-added event; optionally matching the name of the tty to limit, e.g.

  tty-added tty[1-6]

• Set goal to stop on the tty-removed event; matching the first argument from the start event, e.g.

  tty-removed $1

• Instance can be destroyed once stopped; though this might not be desirable, since we might want to be able to start it again.
• We'll need to keep the emission record for the start event as long as the instance is active.
• This will almost certainly need to be combined with some other events or state, for example when there is a writable filesystem. These need to not mask out the tty events, which are the "prime" ones as far as this instance is concerned.