fixes to change engines while Ardour is running.

When a new backend is selected, it is loaded to query
available devices etc. This effectively drops the current backend.
if the user chooses to stop the jack-backend, mark it as stopped. Don't display the "..was shutdown .. was not fast enough.." message.
ALSA backend: engine.halted_callback if the process thread dies
ALSA: tentative hack to request audio-device.