Implement FILE responses.

These are currently bugged, because there's no escaping or quoting
on the file, but it'll do for now.
STAT → STATE; ejct → eject.
Add FEATURES response.
Implement END response.

This will allow auto-advancing in upstream clients.
Rename STAT to STATE in code too.
Replace RESPONSES map with array.

There's no point keeping this as a map when the response API is
more or less fixed.
Simplify command handler.

There's no need for a dynamic, map-and-lambda handler given how
few commands playslave++ has.  Replaced with if statements.
Remove another unnecessary lambda.
Some defensive ejecting/stopping.