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>

gcm_test.cc needs to access the internal GCM symbols. This is
unfortunate because it means that they have to be marked OPENSSL_EXPORT
just for this.

To compensate, modes.h is removed and its contents copied into
crypto/modes/internal.h.

Change-Id: I1777b2ef8afd154c43417137673a28598a7ec30e
Reviewed-on: https://boringssl-review.googlesource.com/6360
Reviewed-by: Adam Langley <alangley@gmail.com>

Change-Id: I63c70f93a0f9395673c9fbe01eb5d864a14a48b6
Reviewed-on: https://boringssl-review.googlesource.com/5520
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Matt Braithwaite <mab@google.com>