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.
3/4 stage fallback will come once I've cleared off some of these existing pull requests. This comment has been handled AFAIK.