stand: Search less agressively for UFS super block

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. This also
has the side effect of eliminating some rather spammy messages when UFS
searches devices with filesystems that are not UFS...

Sponsored by: Netflix
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D36253