MFC of 276737, 322200, 322201, 322271, and 322297

276737: Remove old ioctl use and support
322200: Remove (broken) search for alternate superblocks
322201: Show differences when alternate superblock fails to match
322271: Cleanup for 322200.
322297: Restore fsck_ffs ability to find alternate superblocks

Discussed with: kib, imp
Differential Revision: https://reviews.freebsd.org/D11589
Approved by: re (kib)