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.
must was a mistake. I was trying to stick with RFC meanings re: must Vs should. I left "should" off for varialbes that ended up long and when it didn't seem to be adding anything.

Changed to "should" everywhere.