Add LIBUSB_TRANSFER_ADD_ZERO_PACKET flag to indicate need for ZLP

Add LIBUSB_TRANSFER_ADD_ZERO_PACKET flag to indicate need for ZLP

Some protocols which use USB require an extra zero length data packet
to signal end-of-transfer on bulk endpoints, if the last data packet
is exactly wMaxPacketSize bytes long.

This flag allows applications to inform libusb about this requirement,
so that libusb can handle the issue transparently.

At the moment the new flag is only supported on Linux, and submitting
a transfer with the flag set returns an error at submit time on other
systems. Hopefully implementations will soon follow for other systems.

References #6.