ENH: allow top-level definition of function object name scoping

- this refines commit c233961d45, which added prefix scoping.

  Default is now off (v2106 behaviour).

  The 'useNamePrefix' keyword can be specified on a per function basis
  or at the top-level of "functions".

  ```
      functions
      {
          errors          warn;
          useNamePrefix   true;

          func1
          {
              type  ...;
              useNamePrefix   false;
          }

          func2
          {
              type  ...;
              // Uses current default for useNamePrefix
          }
      }
  ```

tutorials/combustion/XiEngineFoam/kivaTest/system/momentum

@@ -5,6 +5,7 @@ momentum
     type    momentum;
     libs    (fieldFunctionObjects);
     log     true;
+    useNamePrefix   true;
 
     writeControl    writeTime;
     // executeInterval 10;