CGD autoconfig, similar to what RAIDframe does?

I think this requires a layout change, with a cgd header and then the
contents.  But, this would enable checking that the key was correct
without making assumptions about the plaintext, and storing the
algorithm (everything but the key) in the header.  The autoconfig
could then prompt for a key, and try to use the same key for other
autoconfig'd partitions.  The key could be a passphrase protecting the
actual key, so you could have different / and /usr keys, but unlock
them from the same passphrase.

-- 
        Greg Troxel <gdt@ir.bbn.com>