Abstract out transport -- libusb and hidapi

[mcuee]

I am not so sure how difficult this is. I can see that the transport mechanisms are kind of abstracted out to a certain extent but not that clear. It would be good to improve the situation if possible.

Maybe serial port and parallel port are better asbstracted out but I did not look so deep into them. Parallel port is kind of dead anyway.

Example:

1. HIDAPI: the following code is there but it does not seem to be widely used by the programmers, rather the programmers are still using <hidapi/hidapi.h> directly.
   https://github.com/avrdudes/avrdude/blob/main/src/usb_hidapi.c (probably this is not sufficient)

2. libusb-0,1 API: the following code is there but I am not that so sure how widely it is used.
   https://github.com/avrdudes/avrdude/blob/main/src/usb_libusb.c

And then there are quite some fall back codes. I understand the needs to support as wide platforms as possible but I guess it does add to the complexity of the codes.

1. usbasp -- try to use libusb-1.0 API, if not present then fall back to libusb-0.1 API
2. HIDAPI based programmer -- use HIDAPI first, if not present then fall back to libusb
3. PICKit 2 -- use libusb or Win32 API, it would be good to use HIDAPI instead and only fall back when HIDAPI is not present.

[mcuee]

Even though this is a nice to have enhancement, but it is not essential. So I will close this one for now.