[TLS] consensus on backwards compatibility changes

[Dave Garrett <[e-mail]>]

Per Brian's suggestion, I've split the topic of full prohibition of SSL v2 
CLIENT-HELLO usage into its own issue, as there is clearly no consensus on this 
yet. It would be really nice if some real-world stats on how much this is 
actually used could be provided. (I think continued acceptance of it is 
illegitimate, but hard data is harder to argue with)

Issue #113 Prohibit SSL v2 CLIENT-HELLO entirely
https://github.com/tlswg/tls13-spec/issues/113

I have heard notable support for this, including Eric, but I do concede there 
might be too much desire for infinite backwards compatibility to reach consensus. 
I'm still hopeful we can come to an agreement that two decades is a sufficient 
deprecation period. :/

I have two PRs for the backwards compatibility section.

The first has all SSL backwards compatibility changes. In addition to RFC 6176 
SSL 2 language, I've added SSL 3 negotiation prohibition as per Martin's 
suggestion (based on the current I-D). The v2 hello documentation is cut out, 
but seeing as it's only valid for prior versions, prior RFCs can be referenced 
if needed.

PR #105 remove SSL 2 backwards compatibility section & prohibit SSL negotiation
https://github.com/tlswg/tls13-spec/pull/105

The second PR has more general backwards compatibility section improvements, 
prohibition of RC4 (also based on its current I-D), and Brian's ClientHello 
version freeze proposal. Currently, I have the ClientHello version listed as a 
"MUST", but I leave for the possibility that a "SHOULD" might be appropriate if 
other tactics for dealing with buggy servers should continue to be permitted. 
Feedback from Microsoft, Oracle, or anyone else implementing alternate 
workarounds here would be greatly appreciated.

PR #107 revise backwards compatibility & fix ClientHello version
https://github.com/tlswg/tls13-spec/pull/107

Note that the SSL3 & RC4 language based on current drafts is of course 
contingent on those being passed successfully, though this is generally 
expected.

Both PRs update the Informative References as needed, which I've hopefully done 
correctly.


Dave