Stamp out exFAT warnings at the root

Nobody has claimed this yet.

Since adding the exFAT driver, we get spurious warnings about exFAT missing superblocks when trying to detect a partition type. In particular, this happens a lot when trying to find /boot, but it can happen in other cases too.

I made several changes to try to eliminate them in #1602 - tried exFAT after iso9660, tried to exclude bios-grub, LUKS, and LVM partitions, etc.

@tlaurion is making another change in #1784 - exclude very small partitions.

These help, but they still won't be enough. It's looking like a whack-a-mole game to try to eliminate every cause of these warnings. `mount` detects filesystem type just by trying all filesystems, exFAT is the oddball here because every other filesystem just fails to mount without a warning to facilitate this behavior.

As we've discussed before, spurious warnings are a real issue because they hide the causes of real problems and reduce confidence that Heads is working correctly. IMO, we need to eliminate the warning completely, my ideas are either:
* patch the warning out of the kernel
* stop dumping kernel warnings to the console

Review the exFAT partition-detection behavior and the changes referenced in #1602 and #1784 first. Decide whether the warning should be removed in the kernel or whether kernel warnings should stop being printed to the console, then verify that exFAT probing no longer emits spurious warnings without hiding real problems.

Written by the indexing model from the issue text.