kboot: Probe all disks and partitions for a kernel

Guess where to boot from when bootdev= isn't on the command line or
other config. Search all the disks and partitions for one that looks
like it could be a boot partition (same as we do when probing
zpools). Return the first one we find.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D38319