MEM_READ was returning the wrong data for the memory
register being read. This did not impact the software
because the returned value was not being used. It is
now fixed to return the correct data.

Thanks to Marcos Faustino <mrfaustino@pede.com.br> for
pointing this out!

When trying to write a file which has size multiple of
block_size, the last write would try to write 0 bytes
which caused the ICDI firmware to hang. Now checking and
skipping if that's the case.

Verification was broken since #8aea8 because comparison was
not taking into account the first + in the packet. Also
verification errors were not returning for the caller.

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().