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/primitives/block.cpp

@@ -112,11 +112,12 @@ uint256 CBlock::CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMer
 std::string CBlock::ToString() const
 {
     std::stringstream s;
-    s << strprintf("CBlock(hash=%s, ver=%d, hashPrevBlock=%s, hashMerkleRoot=%s, nTime=%u, nBits=%08x, nNonce=%s, vtx=%u)\n",
+    s << strprintf("CBlock(hash=%s, ver=%d, hashPrevBlock=%s, hashMerkleRoot=%s, hashReserved=%s, nTime=%u, nBits=%08x, nNonce=%s, vtx=%u)\n",
         GetHash().ToString(),
         nVersion,
         hashPrevBlock.ToString(),
         hashMerkleRoot.ToString(),
+        hashReserved.ToString(),
         nTime, nBits, nNonce.ToString(),
         vtx.size());
     for (unsigned int i = 0; i < vtx.size(); i++)