Improve headless Ardour somewhat

Improve headless Ardour somewhat

* Allow to exit cleanly (using access-action)
* Terminate when the backend is halted
* Handle various commandline arguments as advertised
  - print version
  - disable hardware optimization
  - set client-name
  - prevent port-connections
* Use JACK -- previously hardour only worked if
  Ardour was compiled with JACK as *only* backend.
  Other backends are listed before JACK, set_default_backend()
  never picks JACK unless it's the only one.
  Other backends need configuration, or EngineState from config.
  This is currently handled in the GUI's engine-dialog..