freenect_flag should be a real flags enum (raw registers should be coded differently)

This changes `freenect_flag` to be a real flags enum, i.e. one where you can set individual bits in the enum (could be used to set settings in bulk in the future, for example, or to quickly query the set of active flags). It is a fix for [issue #341](https://github.com/OpenKinect/libfreenect/issues/341).