Now that Schnorr sigs are there, would be nice to disambiguate both ways. Changes:
DoSign -> DoSignECDSA
PushSig -> PushSigECDSA
PushDataSig -> PushDataSigECDSA
(I left sign_multisig untouched even though it's ECDSA-only; there is no Schnorr counterpart.)