Port ssl3_get_key_exchange to CBS.

Also tidy up some variable names and update RSA_verify call for it no longer
returning -1. Add CBS helper functions for dealing with C strings.

Change-Id: Ibc398d27714744f5d99d4f94ae38210cbc89471a
Reviewed-on: https://boringssl-review.googlesource.com/1164
Reviewed-by: Adam Langley <agl@google.com>
5 files changed