ENH: improve consistency of fileName handling windows/non-windows (#2057)

- wrap command-line retrieval of fileName with an implicit validate.

  Instead of this:
      fileName input(args[1]);
      fileName other(args["someopt"]);

  Now use this:
      auto input = args.get<fileName>(1);
      auto other = args.get<fileName>("someopt");

  which adds a fileName::validate on the inputs

  Because of how it is implemented, it will automatically also apply
  to argList getOrDefault<fileName>, readIfPresent<fileName> etc.

- adjust fileName::validate and clean to handle backslash conversion.
  This makes it easier to ensure that path names arising from MS-Windows
  are consistently handled internally.

- dictionarySearch: now check for initial '/' directly instead of
  relying on fileName isAbsolute(), which now does more things

BREAKING: remove fileName::clean() const method

- relying on const/non-const to control the behaviour (inplace change
  or return a copy) is too fragile and the const version was
  almost never used.

  Replace:
      fileName sanitized = constPath.clean();

  With:
      fileName sanitized(constPath);
      sanitized.clean());

STYLE: test empty() instead of comparing with fileName::null

applications/utilities/surface/surfaceSubset/surfaceSubset.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2015-2020 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -67,15 +67,13 @@ int main(int argc, char *argv[])
     argList args(argc, argv);
 
     Info<< "Reading dictionary " << args[1] << " ..." << endl;
-    IFstream dictFile(args[1]);
+    IFstream dictFile(args.get<fileName>(1));
     dictionary meshSubsetDict(dictFile);
 
     Info<< "Reading surface " << args[2] << " ..." << endl;
+    meshedSurface surf1(args.get<fileName>(2));
 
-    meshedSurface surf1(args[2]);
-
-    const fileName outFileName(args[3]);
-
+    const auto outFileName(args.get<fileName>(3));
 
     Info<< "Original:" << endl;
     surf1.writeStats(Info);
@@ -217,7 +215,7 @@ int main(int argc, char *argv[])
     {
         const dictionary& surfDict = meshSubsetDict.subDict("surface");
 
-        const fileName surfName(surfDict.get<fileName>("name"));
+        const auto surfName(surfDict.get<fileName>("name"));
 
         const volumeType::type volType =
         (