MFC r322322: Expose device caps as libusb_bos_descriptor::dev_capability

Some libusb consumers in Linux-land (in this case, libusb4java) expect a
dev_capability member that they can use to enumerate the device
capabilities.

No particular layout is expected of this, just that it can be traversed
using the bLength member until bNumDeviceCapabilities are read and that the
consumer may then use one of the libusb_get_*_descriptor methods to extract
specific (usb 2.0 vs. ss) capability information.