Updates to UFS/FFS superblock integrity checks when reading a superblock.

Further updates based on analysis of the way the fields are used
in the various filesystem macros defined in fs.h.

Eliminate several checks for non-negative values where the fields
are checked for specific values. Since these specific values are
non-negative, if the value is a verified positive value then it
cannot be negative and such a check is redundant and unnecessary.

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