Since the switch to GPT disk labels, fsck for UFS/FFS has been
unable to automatically find alternate superblocks. This checkin
places the information needed to find alternate superblocks to the
end of the area reserved for the boot block.

Filesystems created with a newfs of this vintage or later will
create the recovery information. If you have a filesystem created
prior to this change and wish to have a recovery block created for
your filesystem, you can do so by running fsck in forground mode
(i.e., do not use the -p or -y options). As it starts, fsck will
ask ``SAVE DATA TO FIND ALTERNATE SUPERBLOCKS'' to which you should
answer yes.

Discussed with: kib, imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11589