ENH: support search modes for etcFiles()

- similar to the foamEtcFile script -mode=... option, the specific
  search location (user/group/other) can now also specified for
  string expansions and as a numerical value for etcFile()

  For example, if searching for group or other (project) controlDict,
  but not wishing to see the user controlDict:

    1.  foamEtcFile -mode=go controlDict

    2.  fileName dictFile("<etc:go>/controlDict");
        dictFile.expand();

    3.  etcFile(controlDict, false, 0077);

  The default behaviour for searching all contexts is unchanged.

    1.  foamEtcFile controlDict

    2.  fileName dictFile("<etc>/controlDict");
        dictFile.expand();

    3.  etcFile(controlDict);

applications/utilities/preProcessing/createZeroDirectory/createZeroDirectory.C

@@ -240,12 +240,13 @@ int main(int argc, char *argv[])
 
     fileName baseDir
     (
-        "${WM_PROJECT_DIR}/etc/caseDicts/createZeroDirectoryTemplates"
+        args.opt<fileName>
+        (
+            "templateDir",
+            // Default is from PROJECT/etc directory
+            "<etc:o>/caseDicts/createZeroDirectoryTemplates"
+        )
     );
-    if (args.found("templateDir"))
-    {
-        baseDir = args["templateDir"];
-    }
 
     baseDir.expand();
     baseDir.toAbsolute();