use const references where appropriate

src/main.cpp

@@ -693,7 +693,7 @@ bool CheckFinalTx(const CTransaction &tx)
 /**
  * Check transaction inputs to mitigate two
  * potential denial-of-service attacks:
- * 
+ *
  * 1. scriptSigs with extra data stuffed into them,
  *    not consumed by scriptPubKey (or P2SH script)
  * 2. P2SH scripts with a crazy number of expensive
@@ -2148,7 +2148,7 @@ static int64_t nTimeFlush = 0;
 static int64_t nTimeChainState = 0;
 static int64_t nTimePostConnect = 0;
 
-/** 
+/**
  * Connect a new block to chainActive. pblock is either NULL or a pointer to a CBlock
  * corresponding to pindexNew, to bypass loading it again from disk.
  */
@@ -3696,7 +3696,7 @@ void static CheckBlockIndex()
 // CAlert
 //
 
-string GetWarnings(string strFor)
+std::string GetWarnings(const std::string& strFor)
 {
     int nPriority = 0;
     string strStatusBar;