use const references where appropriate

src/main.h

@@ -207,7 +207,7 @@ void PartitionCheck(bool (*initialDownloadCheck)(), CCriticalSection& cs, const
 /** Check whether we are doing an initial block download (synchronizing from disk or network) */
 bool IsInitialBlockDownload();
 /** Format a string that describes several potential problems detected by the core */
-std::string GetWarnings(std::string strFor);
+std::string GetWarnings(const std::string& strFor);
 /** Retrieve a transaction (from memory pool, or from disk, if possible) */
 bool GetTransaction(const uint256 &hash, CTransaction &tx, uint256 &hashBlock, bool fAllowSlow = false);
 /** Find the best known block, and make it the tip of the block chain */