crypto/tls: implement FALLBACK_SCSV

[bradfitz]

Per @agl, Go 1.5 should have FALLBACK_SCSV support.

[titanous]

Specifically, support in the TLS client. The server got support in e5624ed.

[titanous]

Hmm, now I'm confused. From the release notes:

(The Go client does not support fallback at all, so it is not vulnerable to those attacks.)

[agl]

Hey cool, it looks like I landed this in 1.4 in the form of e5624ed so it's already done.

[bgentry]

on a related note, a bug that broke TLS_FALLBACK_SCSV out-of-the-box was fixed today: https://go-review.googlesource.com/#/c/1776/

In order to make this work on 1.4, you need to specify a MaxVersion in your tls.Config.