I screwed up the |EVP_CIPHER| parameters for XTS when I first imported
it, and there were no tests to catch it.  (The problem was that
|EVP_CIPH_XTS_MODE| means “the key size is actually twice what it says
here.”)

With these changes, OpenSSL's tests pass.

(Along the way, make a few other things about XTS slightly less
decrepit.)

Change-Id: Icbfbc5e6d532d1c132392ee366f9cab42802d674
Reviewed-on: https://boringssl-review.googlesource.com/6529
Reviewed-by: Adam Langley <agl@google.com>