scripted-diff: put ECDSA in name of signature functions

In preparation for adding Schnorr versions of `CheckSig`, `VerifySignature`, and
`ComputeEntry`, give them an ECDSA specific name.

-BEGIN VERIFY SCRIPT-
sed -i 's/CheckSig(/CheckECDSASignature(/g' $(git grep -l CheckSig ./src)
sed -i 's/VerifySignature(/VerifyECDSASignature(/g' $(git grep -l VerifySignature ./src)
sed -i 's/ComputeEntry(/ComputeEntryECDSA(/g' $(git grep -l ComputeEntry ./src)
-END VERIFY SCRIPT-

src/pubkey.h

@@ -169,7 +169,7 @@ public:
     /*
      * Check syntactic correctness.
      *
-     * Note that this is consensus critical as CheckSig() calls it!
+     * Note that this is consensus critical as CheckECDSASignature() calls it!
      */
     bool IsValid() const
     {