Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/checkqueue.h

@@ -131,7 +131,7 @@ public:
     boost::mutex ControlMutex;
 
     //! Create a new check queue
-    CCheckQueue(unsigned int nBatchSizeIn) : nIdle(0), nTotal(0), fAllOk(true), nTodo(0), fQuit(false), nBatchSize(nBatchSizeIn) {}
+    explicit CCheckQueue(unsigned int nBatchSizeIn) : nIdle(0), nTotal(0), fAllOk(true), nTodo(0), fQuit(false), nBatchSize(nBatchSizeIn) {}
 
     //! Worker thread
     void Thread()