Bug 574139 – There is no way to identify 'user_data' arguments in the callback signature

Bug 574139 – There is no way to identify 'user_data' arguments in the callback signature

Add a heuristic to the scanner that flags arguments of callbacks that
are named 'user_data' (exact match) and have a 'any (void*) type.

These arguments are marked by setting the 'closure' field of ArgBlob
to the index of themselves.

Signed-off-by: Andreas Rottmann <a.rottmann@gmx.at>