SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Unfortunately, it appears that using libusb's synchronous transfer API
from several threads causes nasty race conditions in event handling and
can lead to deadlocks, despite the fact that libusb's synchronous API
is documented to be perfectly fine to use from several threads (only
the manual polling functionality is supposed to require special
precautions).
Since usbdk was the only real reason for using a single libusb context
and since usbdk (currently) has so many issues with Dolphin, I think
dropping support for it in order to fix other backends is acceptable.
* Simplifies libusb context usage and allows us to set options for
all contexts easily. Notably, this lets us enable usbdk support
in libusb, which is now opt-in in the latest version.
* Moves the libusb config descriptor wrapper class to LibusbUtils too
since that could easily be reused.
* Moves device listing to LibusbUtils too and add a lock around it
as some libusb backends are not thread safe.
* Consequences: only a single context and a single event handling
thread is used now, which is more efficient.
