Anti-transaction replay in a hardfork

The purpose of this change would be to maximize fork divergence, and it was suggested that any sensible proposal for a hard fork should include such a change to the sighash. However, Gavin Andresen cautioned that compatibility with existing transaction-signing software and hardware should be considered. He also emphasized the importance of supporting old-sighash transactions in order to facilitate a smooth transition of wallet software and hardware, and to support those who may have older hardware wallets locked away in safe deposit boxes.