core: Add a transfer type for bulk streams.

This transfer type is used by the backend to determine if a transfer is
either a bulk stream transfer or an ordinary bulk transfer. The flag is
being added because there is no other reliable way to determine is a
transfer is a stream without breaking ABI compatibility.
darwin: Add support for bulk stream transfers.

This code has not been completely tested at this time but should work.
xcode: silence Xcode 5 warning