These three cases dovetail with other places in the code where we use
or set D_PARTISGPT when we mean that the partitioning scheme is
GPT. Use this #define to make the code easier to undertand.
NOTE: My reading of grep -r 255 src/stand says I caught them all. Prove me wrong.
Bonus question: Do we use a different #define that happens to be 255 when we mean this one. I don't think so.