Use D_PARTISGPT rather than bare 255

[imp]

Summary

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.

Test Plan

md5 says the .o is the same before and afer :).

[tsoome]

LGTM