Linux: Return ERROR_NOT_FOUND in detach-kernel-driver if usbfs is already bound

Linux: Return ERROR_NOT_FOUND in detach-kernel-driver if usbfs is already bound

* Currently applications for devices which only are accessed from userspace
  can use claim / release interface to make sure they don't get in each others
  way.
* The same however does not work for applications which first need to detach
  a "native" / in kernel driver, as this detach will not only detach native
  drivers but also the usbfs driver, thus stealing the device from another
  userspace / libusbx app.
* This patch fixes libusb_detach_kernel_driver to only detach "real" kernel
  drivers and not the special usbfs driver used for userspace access to
  USB devices. If the usbfs driver is found LIBUSB_ERROR_NOT_FOUND will be
  returned to indicate no driver was detached.