libusb support

Like the vendor GUI, this CLI tool currently requires the ft60x kernel driver which isn't upstream. Loading an out-of-tree kernel module is not always on option. Fortunately talking to the FT601 to the extent required to get usb2sniffer working is pretty straightforward (given the kernel driver as a starting point which is presumably based on reverse engineering of the closed-source FTDI driver) and can be done from user space using libusb (on Linux at least; no idea about Windows).

I've got it working locally but before I clean it up and send a pull request I'd like to discuss the approach first.

There are two topics worth discussing right now:

1. Whether to make libusb support optional

The build infrastructure is currently pretty bare-bones with no configuration or dependency detection support. Given that libusb 1.0 is readily available in pretty much all distros I'd prefer to keep it that way and simply hard-depend on libusb (on Linux).

2. How to select libusb mode and whether to support multiple FT601s

I currently take anything starting with `/` as a path to the ft60x kernel driver device node; otherwise the string is simply ignored and the first FT601 found by libusb is used. This is a bit hacky but straightforward to implement with no changes to code outside of `fops.c` / `fops.h` and addresses the most common use case (few people will have multiple USB analysers or other FT601 based devices connected).