inc, src: PostDispositionWhenNecessaryOnly

- Rename PostDispositionForDirOnly to PostDispositionWhenNecessaryOnly

- Implement PostDispositionWhenNecessaryOnly across the board

tst/memfs-dotnet/Program.cs

@@ -282,6 +282,7 @@ namespace memfs
             Host.ReparsePointsAccessCheck = false;
             Host.NamedStreams = true;
             Host.PostCleanupWhenModifiedOnly = true;
+            Host.PostDispositionWhenNecessaryOnly = true;
             Host.PassQueryDirectoryFileName = true;
             Host.ExtendedAttributes = true;
             Host.WslFeatures = true;