Add a 256-bit reserved field to the block header

This field has no defined semantics. While it was added as a result of
discussions about merged mining in #724, this field will not necessarily ever be
used for that purpose.

src/miner.cpp

@@ -354,6 +354,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
 
         // Fill in header
         pblock->hashPrevBlock  = pindexPrev->GetBlockHash();
+        pblock->hashReserved   = uint256();
         UpdateTime(pblock, Params().GetConsensus(), pindexPrev);
         pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock, Params().GetConsensus());
         pblock->nNonce         = uint256();