Struct page cleanup

I've added the blank line as requested. I think that improves things for people following along at home, but I'm not sure it would have fit on the projector's screen :-)

My slot wasn't focused on the flags, so I didn't fill in all the rest of the details; I juts handwaved at it because everybody else in the room knew what other things were in those bits. I've filled in the fields that are commonly stored in those bits; the problem is that they're all dependent on various CONFIG options and I haven't taken the trouble to figure out likely values in order to size the fields to realistic number of bits. If you can make sense of page-flags-layout.h and the rest of the generating machinery, please let me know. The first problem you'll face is that whoever did the ASCII art in those files was a fan of big-endian bit numbering (probably from IBM).