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.
weird that you use three names for this thing:

sendTlsFallbackIndicator
mustSendTlsFallbackIndicator
shouldSendTlsFallbackIndicator

Pick one?