Update standard superblock when successful using an alternate superblock.

Historically fsck_ffs(8) would only use alternate superblocks when
running in manual mode. When the standard superblock fails, it now
tries to find and use a backup superblocks even when running in `preen'
mode. If an alternate superblock is found and the filesystem is
successfully cleaned up using it, write the alternate superblock
back to the standard superblock so that the filesystem can be
subsequently mounted and used.

Reported by: Peter Holm
Tested by: Peter Holm
Sponsored by: The FreeBSD Foundation