Add ability to suppress UFS/FFS superblock check-hash failure messages.

When reading UFS/FFS superblocks that have check hashes, both the kernel
and libufs print an error message if the check hash is incorrect. This
commit adds the ability to request that the error message not be made.
It is intended for use by programs like fsck that wants to print its
own error message and by kernel subsystems like glabel that just wants
to check for possible filesystem types.

This capability will be used in followup commits.

Sponsored by: Netflix