dissect-image: optionally, reference dissected partition device nodes by diskseq

dissect-image: optionally, reference dissected partition device nodes by diskseq

This is useful to make the dissection logic at boot a bit safer, as we
can reference device nodes by diskseq.

This locks down dissection a bit, since it makes it harder to swap out
the backing device between the time we dissected and validated it, until
we actually mounted it.

This is not complete though, as /bin/mount would have to verify the
diskseq after opening the diskseq symlink again.

See: https://github.com/util-linux/util-linux/issues/1786