Ignore some Xcode cruft
Use timeout for sync_transfer_wait_for_completion
Purely cosmetic changes to distinguish itransfers from transfers consistently
Darwin supports LIBUSB_TRANSFER_ADD_ZERO_PACKET
Fix logic error when backend handles timeouts
Get the Device Descriptor without opening the device
Device Not Open differs from No Device; other error messages.
Backend handles timeouts.
Rationalize function names and other cosmetic changes.
Darwin: avoiding opening device to get descriptor
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