Moved checkpoints out of main, to prep for using them to help prevent DoS attacks

src/checkpoints.h

@@ -0,0 +1,22 @@
+// Copyright (c) 2011 The Bitcoin developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file license.txt or http://www.opensource.org/licenses/mit-license.php.
+#ifndef BITCOIN_CHECKPOINT_H
+#define  BITCOIN_CHECKPOINT_H
+
+class uint256;
+
+//
+// Block-chain checkpoints are compiled-in sanity checks.
+// They are updated every release or three.
+//
+namespace Checkpoints
+{
+    // Returns true if block passes checkpoint checks
+    bool CheckBlock(int nHeight, const uint256& hash);
+
+    // Return conservative estimate of total number of blocks, 0 if unknown
+    int GetTotalBlocksEstimate();
+}
+
+#endif