Adding TLS_FALLBACK_SCSV support

This is an attempt to upstream changes made in Android
for POODLE. I have tried to be more sympathetic here to APIs
and make it optional.

This is early / incomplete, but I'm sending it out for feedback in
case you get a chance to look before Monday.
It should be on by default, and there shouldn't be any API to turn it on or turn it off. If your platform doesn't support it, you don't get the feature. Otherwise you do. We tried our best.

You're right about folks using `COMPATIBLE_TLS` only. Yuck. That's too bad. Ultimately I suppose it depends on how we do 3/4 stage fallback. Is that going to be `MODERN_TLS`, `TLS_2006`, and `TLS_1999` ?