[power] pmdisk Cleanups

[power] pmdisk Cleanups

- Use statically allocated header and info structures.

- Make sure they're packed and page-aligned.

- Make read_swapfiles() return an error if we don't find a valid swap device.

- Store pagedir swap entries in pmdisk_info, eliminating need for linked-list
  at the end of each pages. Greatly simplifies code paths for reading/writing.

- Remove several passed-around swap entries.

- Add full utsname struct to pmdisk_info.

- Add some better header checking and debugging information.