Unify getPortName() across drivers

On OS X it just returns the port name, so if you open a virtual port with `input->openVirtualPort("foo bar')` then `input->getPortName(1)` it returns `foo bar`. 

Looking at the Windows code it seems to just return the name.

Under ALSA it returns `RtMidi Input Client:0` making all but impossible to match to the name given when opening it. I modified this code to only return the port name.

I'm not quite clear what the Jack code returns.i