darwin: add support for the LIBUSB_TRANSFER_ADD_ZERO_PACKET transfer flag.

The zero-length packet is sent by the async event thread if all the following
conditions are met:

1) The packet size is not a multiple of maxPacketSize, and
2) the transfer completed successfully.

References #6.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
2 files changed