lowl-vcom: Add support for auto scanning (-S auto) on Mac OS X

Uses I/O Kit to enumerate USB devices
https://developer.apple.com/library/mac/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/Introduction/Introduction.html
http://en.wikipedia.org/wiki/I/O_Kit
Yes. Darwin is just the OSS kernel for OS X (XNU). I/O Kit is the I/O framework for the XNU kernel in OS X and NeXTSTEP.