[trac550] make sure the user defined flag bit is the MSB.

the actual bit position isn't a big deal right now, but the intent would be
a bit clearer than an odd intermediate position (and, in fact, using the
MSB was the original intent; the previous value was a typo)

src/lib/datasrc/rbtree.h

@@ -121,7 +121,7 @@ public:
     /// set to on by the \c setFlag() method.
     enum Flags {
         FLAG_CALLBACK = 1, ///< Callback enabled. See \ref callback
-        FLAG_USER1 = 0x8000000U ///< Application specific flag
+        FLAG_USER1 = 0x80000000U ///< Application specific flag
     };
 private:
     // Some flag values are expected to be used for internal purposes