Changing parameters on renegotiation makes all our APIs confusing. This
one has no reason to change, so lock it down. In particular, our
preference to forbid Token Binding + renego may be overridden at the
IETF, even though it's insane. Loosening it will be a bit less of a
headache if EMS can't change.

https://www.ietf.org/mail-archive/web/unbearable/current/msg00690.html
claims that this is already in the specification and enforced by NSS. I
can't find anything to this effect in the specification. It just says
the client MUST disable renegotiation when EMS is missing, which is
wishful thinking. At a glance, NSS doesn't seem to check, though I could
be misunderstanding the code.

Nonetheless, locking this down is a good idea anyway. Accurate or not,
take the email as an implicit endorsement of this from Mozilla.

Change-Id: I236b05991d28bed199763dcf2f47bbfb9d0322d7
Reviewed-on: https://boringssl-review.googlesource.com/10721
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>