Accept any sequence of PUSHDATAs in OP_RETURN outputs

Previously only one PUSHDATA was allowed, needlessly limiting
applications such as matching OP_RETURN contents with bloom filters that
operate on a per-PUSHDATA level. Now any combination that passes
IsPushOnly() is allowed, so long as the total size of the scriptPubKey
is less than 42 bytes. (unchanged modulo non-minimal PUSHDATA encodings)

Also, this fixes the odd bug where previously the PUSHDATA could be
replaced by any single opcode, even sigops consuming opcodes such as
CHECKMULTISIG. (20 sigops!)

src/policy/policy.cpp

@@ -49,7 +49,9 @@ bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType)
             return false;
         if (m < 1 || m > n)
             return false;
-    }
+    } else if (whichType == TX_NULL_DATA &&
+               (!GetBoolArg("-datacarrier", true) || scriptPubKey.size() > nMaxDatacarrierBytes))
+          return false;
 
     return whichType != TX_NONSTANDARD;
 }