x/crypto/ssh: hide msgNewKeys in the transport layer.

This ensures that extraneous key exchanges cannot confuse application
level code.

Change-Id: I1a333e2b7b46f1e484406a79db7a949294e79c6d
Reviewed-on: https://go-review.googlesource.com/22417
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Adam Langley <agl@golang.org>
5 files changed