Generic anti-replay protection using Script.

Proposed a script check that ties a payment to one particular chain, so it can't be copied onto another chain after a split.

If Bitcoin ever split into two lasting chains, a payment made on one could be replayed on the other. This check would make a payment valid only if a chosen block is in the chain's history. It could also help a wallet stay safe when an unconfirmed payment it received is double-spent.