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/preProcessing/changeDictionary/changeDictionary.C

@@ -540,7 +540,7 @@ int main(int argc, char *argv[])
                 "boundary",
                 runTime.findInstance
                 (
-                    polyMesh::regionName(regionName)/polyMesh::meshSubDir,
+                    polyMesh::meshDir(regionName),
                     "boundary",
                     IOobject::READ_IF_PRESENT
                 ),

applications/utilities/preProcessing/createZeroDirectory/boundaryInfo.C

@@ -54,8 +54,8 @@ Foam::IOPtrList<Foam::entry> Foam::boundaryInfo::readBoundaryDict
         IOobject
         (
             "boundary",
-            runTime.findInstance(regionName/polyMesh::meshSubDir, "boundary"),
-            regionName/polyMesh::meshSubDir,
+            runTime.findInstance(polyMesh::meshDir(regionName), "boundary"),
+            polyMesh::meshDir(regionName),
             runTime,
             IOobject::MUST_READ,
             IOobject::NO_WRITE,

applications/utilities/preProcessing/foamUpgradeCyclics/foamUpgradeCyclics.C

@@ -409,15 +409,13 @@ int main(int argc, char *argv[])
     }
     const bool enableEntries = args.found("enableFunctionEntries");
 
-    const word regionName =
-        args.getOrDefault<word>("region", polyMesh::defaultRegion);
-
     fileName regionPrefix;
-    if (regionName != polyMesh::defaultRegion)
     {
-        regionPrefix = regionName;
-    }
+        // Specified region or default region
+        #include "getRegionOption.H"
 
+        regionPrefix = polyMesh::regionName(regionName);
+    }
 
     // Per cyclic patch the new name for this side and the other side
     HashTable<word> thisNames;