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

src/functionObjects/utilities/abort/abort.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -167,11 +167,10 @@ bool Foam::functionObjects::abort::read(const dictionary& dict)
     if (dict.readIfPresent("file", file_))
     {
         file_.expand();
-
-        if (!file_.isAbsolute() && file_.size())
+        if (!file_.empty() && !file_.isAbsolute())
         {
             file_ = time_.globalPath()/file_;
-            file_.clean();
+            file_.clean();  // Remove unneeded ".."
         }
     }
 
@@ -179,7 +178,7 @@ bool Foam::functionObjects::abort::read(const dictionary& dict)
     if (file_.empty())
     {
         file_ = time_.globalPath()/name();
-        file_.clean();
+        file_.clean();  // Remove unneeded ".."
     }
 
     triggered_ = false;

src/functionObjects/utilities/ensightWrite/ensightWrite.C

@@ -175,7 +175,7 @@ bool Foam::functionObjects::ensightWrite::read(const dictionary& dict)
         // Standard postProcessing/ naming
         outputDir_ = time_.globalPath()/functionObject::outputPrefix/name();
     }
-    outputDir_.clean();
+    outputDir_.clean();  // Remove unneeded ".."
 
     return true;
 }

src/functionObjects/utilities/vtkWrite/vtkWrite.C

@@ -236,7 +236,7 @@ bool Foam::functionObjects::vtkWrite::read(const dictionary& dict)
         // Standard postProcessing/ naming
         outputDir_ = time_.globalPath()/functionObject::outputPrefix/name();
     }
-    outputDir_.clean();
+    outputDir_.clean();  // Remove unneeded ".."
 
     return true;
 }