- Move struct pmdisk_header definition to swsusp and change name to struct
  swsusp_header.
- Statically allocate one (swsusp_header), and use it during mark_swapfiles()
  and when checking sig on resume.
- Move check_sig() from pmdisk to swsusp.
- Wrap with swsusp_verify(), and move check_header() there. 
- Fix up calls in pmdisk and swsusp.
- Make new wrapper swsusp_close_swap() and call from write_suspend_image().
- Look for swsusp_info info in swsusp_header.swsusp_info, instead of magic
  location at end of struct.