OSspecific: altered file tests

- removed the previously added fileName type(), isDir(), isFile() methods.

- added optional bool parameter to isFile() function to explicitly
  enable/disable the check for gzip files.

- fixed minor bugginess where the default usage of isFile() would result in
  false positive matches.

- be slightly more stringent and use isDir() and isFile() instead of
  exists() function when we actually know the expected type.

applications/utilities/mesh/manipulation/splitMeshRegions/splitMeshRegions.C

@@ -627,7 +627,7 @@ autoPtr<mapPolyMesh> createRegionMesh
         Info<< "Testing:" << io.objectPath() << endl;
 
         if (!io.headerOk())
-        //if (!exists(io.objectPath()))
+        // if (!exists(io.objectPath()))
         {
             Info<< "Writing dummy " << regionName/io.name() << endl;
             dictionary dummyDict;