ENH: minor simplifications for handling region meshes

- static version of polyMesh::meshDir(), which takes a region name

      polyMesh::meshDir(regionName)
  vs
      polyMesh::regionName(regionName)/polyMesh::meshSubDir

STYLE: use polyMesh::regionName(..) instead of comparing to defaultRegion

STYLE: use getOrDefault when retrieving various -region options

FIX: polyMesh::dbDir() now checks registry name, not full path (#3033)

applications/utilities/finiteArea/makeFaMesh/findMeshDefinitionDict.H

@@ -13,6 +13,19 @@ License
 Description
     Search for the appropriate faMeshDefinition dictionary...
 
+Required Classes
+    - Foam::polyMesh
+    - Foam::IOdictionary
+
+Required Variables
+    - regionName [word]
+    - args [argList]
+    - runTime [Time]
+
+Provided Variables
+    - meshDefDict [IOdictionary]
+    - meshDictPtr [autoPtr<IOdictionary>]
+
 \*---------------------------------------------------------------------------*/
 
 const word dictName("faMeshDefinition");
@@ -21,7 +34,7 @@ autoPtr<IOdictionary> meshDictPtr;
 
 {
     fileName dictPath;
-    const word& regionDir = polyMesh::regionName(regionName);
+    const word& regionDir = Foam::polyMesh::regionName(regionName);
 
     if (args.readIfPresent("dict", dictPath))
     {