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/sampling/probes/probes.C

@@ -208,14 +208,13 @@ Foam::label Foam::probes::prepare()
         // Put in undecomposed case
         // (Note: gives problems for distributed data running)
 
-        fileName probeDir =
+        fileName probeDir
         (
             mesh_.time().globalPath()
           / functionObject::outputPrefix
           / probeSubDir
           / mesh_.time().timeName()
         );
-
         probeDir.clean();  // Remove unneeded ".."
 
         // ignore known fields, close streams for fields that no longer exist

src/sampling/sampledSet/shortestPath/shortestPathSet.C

@@ -1316,13 +1316,13 @@ Foam::shortestPathSet::shortestPathSet
 {
     if (debug)
     {
-        fileName outputDir =
+        fileName outputDir
         (
             mesh.time().globalPath()
           / functionObject::outputPrefix
           / mesh.pointsInstance()
         );
-        outputDir.clean();
+        outputDir.clean();  // Remove unneeded ".."
 
         Info<< "shortestPathSet : Writing blocked faces to "
             << outputDir << endl;