stand: Search less agressively for UFS super block

[imp]

Summary

The boot loader should look in the standard places for the UFS
superblock, but not go too far into the speculative realm. Supress
errors about hash being invalid, which will allow us to boot, even when
the superblock looks good, but fails the hash test. This defers any
policy decisions about booting and/or recovery to userland. As a side
effect, we no longer spam the console with a lot of useless messages
about not being able to find UFS superblocks on devices that do not
contain a UFS filesystem.

Sponsored by: Netflix

[mckusick]

This gets us back to close to what we had previously. You might want to reduce output further by adding the UFS_NOMSG flag, but generally non-UFS systems will not have a valid magic number which will prevent any further messages, so you should only get messages if you have something that is likely to be a UFS filesystem.

[vidwer_fbsdbugs_gmail.com]

I can confirm the console spam for non-UFS, ZFS, installations on bare metal (amd64) is gone.

[imp]

In D36253#823067, @mckusick wrote:

This gets us back to close to what we had previously. You might want to reduce output further by adding the UFS_NOMSG flag, but generally non-UFS systems will not have a valid magic number which will prevent any further messages, so you should only get messages if you have something that is likely to be a UFS filesystem.

My plan was to do this and see how many reports of extra output we get that's not helpful.