Add IsPushOnly(const_iterator pc)

Allows IsPushOnly() to be applied to just part of the script for
OP_RETURN outputs.

src/script/script.h

@@ -589,6 +589,7 @@ public:
     bool IsPayToScriptHash() const;
 
     /** Called by IsStandardTx and P2SH/BIP62 VerifyScript (which makes it consensus-critical). */
+    bool IsPushOnly(const_iterator pc) const;
     bool IsPushOnly() const;
 
     /**