ENH: add optional checkGzip parameter to fileName::type(..) method

- this simplifies use of a unified test for directory or file.

      fileName::Type what = myfile.type(true, true);
      if (what == FILE) ...
      if (what == DIRECTORY) ...

- Use distinct bit values for fileName::Type, for possible use in
  the future.

- related to issue #1121, since we need a more flexible way of
  expanding file or directory.

  An alternative would be to add checkGzip to Foam::exists() and
  Foam::type() functions, but that would make the code there more
  confusing and in the fileHandler classes.