On some machines, some usb responses would be broken
into more that two bulk_requests. Now the reception
of the whole response in handled in a single call to
wait_response.

The new main function just handles argument processing and calls
flasher_flash()

This function, given all arguments, simply flashes the image

This function simply shows the command line arguments

This function finds the USB device with USB vendor/product IDs.
Multiple identical products can be differentiated with the serial
number, if needed.

The matching device, if unique, is referenced and returned.

This error code captures higher-level errors than what libusb returns
alone. It also has a dedicated code for libusb errors.

This happens e.g. when the checksum is incorrect, but not for much else.

Move the variables required for debug code into #ifdef DEBUG,
and delete the completely unused libusb_device *dev in main().