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/surfaceLambdaMuSmooth/surfaceLambdaMuSmooth.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2015 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -156,11 +156,11 @@ int main(int argc, char *argv[])
     );
     argList args(argc, argv);
 
-    const fileName surfFileName = args[1];
-    const scalar lambda = args.get<scalar>(2);
-    const scalar mu = args.get<scalar>(3);
-    const label  iters = args.get<label>(4);
-    const fileName outFileName = args[5];
+    const auto surfFileName = args.get<fileName>(1);
+    const auto lambda = args.get<scalar>(2);
+    const auto mu = args.get<scalar>(3);
+    const auto iters = args.get<label>(4);
+    const auto outFileName = args.get<fileName>(5);
 
     if (lambda < 0 || lambda > 1)
     {
@@ -192,7 +192,7 @@ int main(int argc, char *argv[])
 
     if (args.found("featureFile"))
     {
-        const fileName featureFileName(args["featureFile"]);
+        const auto featureFileName = args.get<fileName>("featureFile");
         Info<< "Reading features from " << featureFileName << " ..." << endl;
 
         edgeMesh feMesh(featureFileName);