Replace repeated GetBoolArg() calls with Checkpoint::fEnabled variable

set once at init time.

src/checkpoints.cpp

@@ -28,6 +28,8 @@ namespace Checkpoints
         double fTransactionsPerDay;
     };
 
+    bool fEnabled = true;
+
     // What makes a good checkpoint block?
     // + Is surrounded by blocks with reasonable timestamps
     //   (no blocks before with a timestamp after, none after with
@@ -74,7 +76,7 @@ namespace Checkpoints
 
     bool CheckBlock(int nHeight, const uint256& hash)
     {
-        if (!GetBoolArg("-checkpoints", true))
+        if (!fEnabled)
             return true;
 
         const MapCheckpoints& checkpoints = *Checkpoints().mapCheckpoints;
@@ -117,7 +119,7 @@ namespace Checkpoints
 
     int GetTotalBlocksEstimate()
     {
-        if (!GetBoolArg("-checkpoints", true))
+        if (!fEnabled)
             return 0;
 
         const MapCheckpoints& checkpoints = *Checkpoints().mapCheckpoints;
@@ -127,7 +129,7 @@ namespace Checkpoints
 
     CBlockIndex* GetLastCheckpoint(const std::map<uint256, CBlockIndex*>& mapBlockIndex)
     {
-        if (!GetBoolArg("-checkpoints", true))
+        if (!fEnabled)
             return NULL;
 
         const MapCheckpoints& checkpoints = *Checkpoints().mapCheckpoints;