Identify each UFS/FFS superblock integrity check as a warning or fatal error.

Identify each of the superblock validation checks as either a
warning or a fatal error. Any integrity check that can cause a
system hang or crash is marked as fatal. Those that may simply
lead to poor file layoutor other less good operating conditions
are marked as warning.

Normally both fatal and warning are treated as errors and prevent
the superblock from being loaded. A new flag, UFS_NOWARNFAIL, is
added. When passed to ffs_sbget() it will note warnings that it
finds, but will still proceed with loading the superblock. Note
that when UFS_NOWARNFAIL is used, it also includes UFS_NOHASHFAIL.

No legitimate superblocks should fail as a result of these changes.