{sign|verify}message replacement

In an email conversation between Karl Johan Alm and Luke Dashjr, the topic of Bitcoin's message signing feature was discussed. Luke Dashjr suggested that the feature should support "proof of funds" as a separate feature in addition to the current "proof I receive at this address" function. He noted that the current message signing feature does not actually prove the presence of funds, which makes it difficult to use for verifying ownership of coins. To accomplish this, the feature would need to be able to sign for multiple inputs. Karl Johan Alm responded by pointing out that such a feature could potentially compromise fungibility and make hot/cold wallet separation impossible. Instead, he suggested that the feature should simply allow users to prove that funds are available rather than indicating specific UTXOs. The current sign message feature only proves that you have received funds, but not that you are currently in possession of them.