- previously hidden as Detail::[IO]FstreamAllocator, now exposed
  directly as [io]fstreamPointer, which allows reuse for
  std::ifstream, std::ofstream wrapping, without the additional
  ISstream, OSstream layers.

  These stream pointers have some characteristics similar to a
  unique_ptr.

- restrict direct gzstream usage to two files (fstreamPointers.C,
  gzstream.C) which improves localization and makes it simpler to
  enable/disable with the `HAVE_LIBZ` define.

  The HAVE_LIBZ define is currently simply hard-coded in the
  Make/options.

  If compiled WITHOUT libz support:
    - reading gz files : FatalError
    - writing gz files : emit warning and downgrade to uncompressed
    - warn if compression is specified in the case controlDict
      and downgrade to uncompressed

ENH: minor updates to gzstream interface for C++11

- support construct/open with std::string for the file names.

CONFIG: provisioning for have_libz detect...