Refine libusb_set_configuration() semantics
Applications will generally want to set a configuration before claiming
interfaces. The problem is that the interface may already be set, and
someone else may have claimed an interface (meaning that all calls to
set_configuration will fail, even if it's for the same configuration).
There are now 2 options:
1. Use the new libusb_get_configuration() to determine active
configuration before calling libusb_set_configuration()
2. Or just call libusb_set_configuration() as usual, which will do
nothing if that configuration is already active.