{sign|verify}message replacement

In an email conversation on March 16, 2018, Greg Sanders asked why not just do a transaction with a FORKID specifically for this. This would allow a node to have a mempool acceptance test that returns true even if the signature is not valid according to Bitcoin consensus but only due to the FORKID. With this approach, any wallet can support this provided generic FORKID support. However, it would require providing an entire transaction rather than just the signature, which may become unwieldy quicker when copy-pasting.