Add constant scriptCode policy in non-segwit scripts

This disables OP_CODESEPARATOR in non-segwit scripts (even in an unexecuted branch), and makes a positive FindAndDelete result invalid. This ensures that the scriptCode serialized in SignatureHash() is always the same as the script passing to the EvalScript.

src/script/script_error.h

@@ -64,6 +64,10 @@ typedef enum ScriptError_t
     SCRIPT_ERR_WITNESS_UNEXPECTED,
     SCRIPT_ERR_WITNESS_PUBKEYTYPE,
 
+    /* Constant scriptCode */
+    SCRIPT_ERR_OP_CODESEPARATOR,
+    SCRIPT_ERR_SIG_FINDANDDELETE,
+
     SCRIPT_ERR_ERROR_COUNT
 } ScriptError;