Audio drivers should not be created if MuseScore is run with "-s"

MuseScore 2, commit daf97e1.

Reproduce:
1. Run MuseScore and select "JACK audio driver" in Preferences-> I/O (assuming you have JACK properly installed and configured).
2. Close and run MuseScore again with "-s": "./mscore -s"
3. Open your patchbay to see the connections.

Expected result: no "mscore" instances/ports exists.
Actual result: "mscore" instance with audio/midi ports is visible (like runned without "-s").

I suspect that we have same situation with other audio drivers.

Additional tasks:
1. Check if sequencer is really not running when MuseScore 2 runned with "-s".
3. Forbid selecting audio driver when MuseScore 2 runned with "-s" by deselecting Preferences-> I/O tab.