ENH: allow space char in fileName is now configurable (#1008)

- having whitespace in fileName can be somewhat fragile since it means
  that the fileName components do not necessarily correspond to a
  'Foam::word'. But in many cases it will work provided that spaces
  are not present in the final portion of the simulation directory
  itself.

    InfoSwitches
    {
        // Allow space character in fileName (use with caution)
        allowSpaceInFileName    0;
    }

- now use doClean=true as default for fileName::validate(). Was false.

  Unlike fileName::clean() this requires no internal string rewrite
  since the characters are being copied.  Also handle any path
  separator transformations (ie, backslash => forward slash) at the
  same time. This makes it resemble the std::filesystem a bit more.