examples: add an option to force a device request for WCID descriptors

* Add option 'w' to force the use of a Device Request rather than an
  Interface Request when querying the WCID OS Extended Properties descriptor.
* This is due to a WinUSB limitation where all Interface Requests have the
  wIndex set to the interface number.
* This assumes that the WCID firmware answers both Device and Interface requests
  equally.
Windows: Avoid potential mismatch in transfer error reporting

The default case in windows_transfer_callback() calls windows_error_str(0)
which will convert the error code returned by GetLastError() to a string.
This currently works because windows_transfer_callback() is either called
with io_result set to NO_ERROR or io_result set to GetLastError().

If windows_transfer_callback() will ever be called with io_result set
differently then the default case might report the wrong error message.

Call windows_error_str(io_result) instead to avoid this.

Closes #151